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T3 Workshop Technology 
for Mechanical Engineering 
Technicians 



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Si Edition 



T. 3 

WORKSHOP 

TECHNOLOGY 

For Mechanical Engineering Technicians 



R. T. Pritchard 

CEng, MI Prod E, Full Tech. Cert. CGLI, 
Teacher's Cert, in Metalwork 

Lecturer in Mechanical Engineering, 

Garretts Green Technical College, Birmingham. 

Examiner for the City and Guilds of London Institute, 

The Union of Educational Institutions, and 

The Welsh Joint Education Committee 



Illustrations by Joy Armon 



II 



HODDER AND STOUGHTON 
LONDON SYDNEY AUCKLAND TORONTO 



The Technical College Series 
Edited by 

E. G. Sterland, JP, MA, BSc(Eng),FIMechE, FRAeS 

Principal, Rolls-Royce Technical College, Bristol 
Consulting Editor 

P. D. Collins, MSc, CEng, FIMechE, FIProdE 

Principal, Worcester Technical College 



WIGAN LP'SURH SERVICES 



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6**0898 




Boards edition isbn o 340 14950 7 
Paperback edition isbn o 340 15762 3 

First published 1965 
Reprinted 1967 
Second edition 1972 
Reprinted 1973, 1975 

Copyright © 1972 R. T. Pritchard 

All rights reserved. No part of this publication may be reproduced or 
transmitted in any form or by any means, electronic or mechanical in- 
cluding photocopy, recording, or any information storage, or retrieval 
system, without permission in writing from the publisher. 

Printed in Great Britain for Hodder and Stoughton Educational, 

a division of Hodder and Stoughton Ltd, St. Paul's House, Warwick Lane, 

London EC4P 4AH by Hazell Watson & Viney Ltd, Aylesbury, Bucks 



Editor's Foreword 



The Technical College Series covers a wide range of technician and craft 
courses, and includes books designed to cover subjects in National Certifi- 
cate and Diploma courses and City and Guilds Technician and Craft 
syllabuses. This important sector of technical education has been the sub- 
ject of very considerable changes over the past few years. The more recent 
of these have been the result of the establishment of the Training Boards, 
under the Industrial Training Act. Although the Boards have no direct 
responsibility for education, their activities in ensuring proper training in 
industry have had a marked influence on the complementary courses 
which Technical Colleges must provide. For example, the introduction 
of the module system of training for craftsmen by the Engineering Industry 
Training Board led directly to the City and Guilds 500 series of courses. 

The Haslegrave Committee on Technician Courses and Examinations 
reported late in 1969, and made recommendations for far-reaching admini- 
strative changes, which will undoubtedly eventually result in new syllabuses 
and examination requirements. 

It should, perhaps, be emphasised that these changes are being made not 
for their own sake, but to meet the needs of industry and the young men and 
women who are seeking to equip themselves for a career in industry. And 
industry and technology are changing at an unprecedented rate, so that 
technical education must be more concerned with fundamental principles 
than with techniques. 

Many of the books in the Technical College Series are now standard 
works, having stood the test of time over a long period of years. Such books 
are reviewed from time to time and new editions published to keep them 
up to date, both in respect of new technological developments and chang- 
ing examination requirements. For instance, these books have had to be 
rewritten in the metric system, using SI units. To keep pace with the rapid 
changes taking place both in courses and in technology, new works are 
constantly being added to the list. The Publishers are fully aware of the 
part that well-written up-to-date textbooks can play in supplementing 
teaching, and it is their intention that the Technical College Series shall 
continue to make a substantial contribution to the development of tech- 
nical education. 

E. G. Ster^and 



Author's Preface 



Few people outside the engineering industry appreciate the contribution of 
modern technology in meeting the demands and needs of our present-day 
civilisation. The study of the scientific principles underlying the art of engin- 
eering manufacture is the study of Workshop Technology, an all-important 
subject when one considers that it is skill in engineering manufacture that 
makes possible a standard of living beyond the wildest expectations of the 
previous generation. 

It is the emergence of the machine tool within the last 150 years that has 
made possible the ability of the civilised world to provide the essentials of 
existence to its own rapidly growing population, and perhaps of greater im- 
portance, to make available modern techniques and processes to the under- 
developed territories of the world. 

The need for efficient and well-trained mechanical engineering tech- 
nicians is great. The stated aim of the scheme of work contained in the 
Mechanical Engineering Technicians' Course is to meet the needs of those 
who aspire to supervisory duties, shop and process control, drawing office 
practice, plant maintenance and other forms of responsibility. 

This class book is written to meet the needs of those engaged on the third 
year of the technicians' course. It is a continuation of the two preceding 
volumes, and once again I have attempted to keep the approach simple and 
direct, making full use of line diagrams to illustrate the text. 

Finally I have been much encouraged by the favourable reception given 
to the preceding volumes of this series, and I am indebted to those who have 
written to me expressing their appreciation of my efforts. I am grateful too, 
to Miss Joy Armon for the diagrams, and to Brian G. Staples, MA, FLA, 
for his continued help and advice in reading the proofs. 
Sutton Coldfield R. T. Pritchard 



IV 



Contents 



i Principles and Applications of Welding i 

Oxy-acetylene welding— flame cutting— arc welding— shielded- 
arc welding— submerged-arc welding— argon arc welding- 
spot welding— seam welding— stitch welding— projection weld- 
ing—flash-butt welding— weld testing. 

2 Measurement 2 5 

End measuring bars— precision rollers— mechanical comparators 
—electrical comparators— optical comparators— toolmaker's 
microscope. 

3 Inspection 54 

Dimensional control— mass production— BS 4500— tolerance 
grades— fundamental deviation— selection of fits— clearance fits 
—transition fits— interference fits— preferred numbers— limit 
gauges— BS 1044— BS 969. 

4 Cutting Tools 79 

Cutting tool materials— high-carbon steel— high-speed steel— 
stellite— cemented carbides— ceramics— cutting speeds— tool 
failure— built-up edge— cratering— forces at tool point— lathe 
dynamometer— radial cutting— tangential cutting— negative 
rake cutting— cutter grinding. 

5 The Centre Lathe 99 

Tool holding— work holding— toolmaker's buttons— faceplate 
balancing— use of centres— mandrels— screw cutting— multi- 
start threads— cutting- tool angles— vertical boring machines- 
duplex boring mill— vertical turret lathe. 

6 Turret and Capstan Lathes I2 3 

Turret lathes— capstan lathes— use of stops— work holding- 
spindle speeds— knee turning toolholder— boring bars— exten- 
sion arms— starting drill and holder— roller-steady turning tool- 
holder— roller-steady ending toolholder— self-opening diehead 
—collapsible tap— example of bar work— machining times. 



VI CONTENTS 

7 Hole Production 147 

Types of drilling machines — hole piercing — drill jigs — gang 
drilling — multi-head drilling — hole broaching— jig boring- 
control of linear worktable movement — horizontal boring — 
essential features of horizontal borer. 

8 Milling Machines 167 

Standard milling techniques — fixed-bed millers — duplex milling 
machines — rotary-table milling — up-cut milling — down-cut mil- 
ling — negative-rake milling — form milling — universal milling 
machine — dividing head — simple indexing — angular indexing 
— compound indexing — differential indexing — spiral milling — 
cam milling. 

Index 1 99 



1 Welding 



i.i The importance of welding 

Welding is essentially a metal joining process. We define it as the art of 
joining two metal parts in such a way that the resultant joint is equivalent in 
composition and characteristics to the metals used. This is shown in fig. i .1 . 
Ideally parts A and B, when welded, are equivalent to the metal bar shown 
at D ; this means that the welded joint has in no way weakened, say, the u.t.s. 
of the welded bar shown at C. This is the true purpose of welding: the pro- 
duction of a homogeneous bond between the two metal parts joined to- 
gether. 




Solid bar 
Welded bar 

Strength of welded bar - strength of solid bar 

Fig. i . i — Essentials of a Welded Joint. 

The rapid and efficient joining of metal parts is an essential requirement 
of modern engineering manufacture. To meet the demands of mass and 
flow production of engineering components, enormous strides have been 
taken in the development of welding techniques within the last 50 years, and 
it is true to state that no other engineering process makes greater use of 
scientific principles than the welding process. 

Two simple examples will serve to illustrate the importance of welding. 
Fig. 1 .2 shows a V pulley, prefabricated from four bright mild-steel sheet 
pressings. 

This pulley is part of the driving mechanism of a mass-produced domestic 
washing machine. Clearly the mild-steel pressings shown as details 1, 2, 3 
and 4 in the sectional view of the pulley in fig. 1 .2 can be rapidly and 
cheaply produced using press tools. 



WORKSHOP TECHNOLOGY I 



Note that the pressings 2 and 3 are welded to pressing 1 , while pressing 4 
is welded to pressing 3. 

The front elevation shows the welding points ; a total of 22 are required to 
join the parts together. The steel bush has a serrated face permitting positive 
drive to the welded pulley, and is a force fit on the phosphor bronze bush. 




Spot welds 
20Q mm diameter j 



MILD STEEL VEE PULLEY 



Fig. 1.2 — Prefabricated V Pulley. 

The alternative method of manufacture is to die cast a pulley in a non- 
ferrous alloy, with subsequent precision machining of the bore. Clearly the 
welding technique, which allows the prefabrication of this pulley, offers very 
great savings, not only in the amount and cost of the material required, but 
also in manufacturing time. 

Fig. 1 .3 shows another important advantage of the utilisation of a welding 
process. The lathe tool illustrated has a high-speed cutting portion, butt 
welded to a tough medium carbon or alloy steel shank. High-speed steel is 
an expensive metal, and the cost of this and similar tools is greatly reduced 
because of the butt welding technique adopted to make use of a cheaper yet 




Medium carbon 
or alloy steel 



utt weld 
High speed steel 

Fig. 1.3.— Butt Welded Lathe Tool. 



PRINCIPLES AND APPLICATIONS OF WELDING 3 

suitable metal for the body of the tool. Most high-speed drills and reamers 
have butt welded shanks of alloy steels. 

1.2 Welding techniques 

Clearly the two previous examples of welding differ greatly in the actual 
technique employed to join together the metal parts. 

The assembly of the pulley shown in fig. 1.2 is brought about by weliding 
at different parts or spots, whilst the joining of a high-speed cutting portion 
to the body of a lathe tool involves the total joining over a fairly large contact 
area. In both cases fusion of the metals results, but the actual technique or 
classification of the weld used depends on the type of joint required, the 
metal thickness, and the area of contact. Metal fusion requires the applica- 
tion of heat, and the heat source is a useful indication of the type of weld 
obtained. 

The following diagram, fig. 1 .4, indicates the main welding techniques or 
processes. Note that two main sources of heat are employed : chemical and 
electrical. Note also that a much greater scope is offered by the electrical 

SOURCE OF HEAT 



ELECTRICAL CHEMICAL 



RESISTANCE 
WELDING 
I 
SPOT ARC 

SEAM 

PROJECTION 

1 



FORGE OXY-ACETYLENE 

WELDING WELDING 



BUTT 
FLASH 



Fig. 1.4.— Line Diagram of Welding Techniques. 



source of heat. If we remember, too, that electricity is readily controlled and 
easily applied, then we will realise that electrical methods of heat supply are 
certain to be much in evidence in the welding techniques or processes used 
in the mass-production of engineering components. Most of these electrical 
methods of welding have been developed within the last few decades mainly 
as a result of the limitations of the oxy-acetylene welding process. 

1.3 Oxy-acetylene welding 

The invention in 1 895 of the oxy-acetylene blowpipe is credited to the 
French thermodynamicist Le Chatelier, and all modern oxy-acetylene 
welding techniques owe their existence to this important development. 



4 WORKSHOP TECHNOLOGY I 

It is known that the primitive engineers of prehistoric times were able to 
forge precious metals into simple ornaments. The principle adopted was the 
same as that used in forge welding at the present time : namely the insertion 
of the parts to be welded into a suitable forge or fire. This meant that forge 
welding was a static process ; the metal had to be taken to the source of heat. 
Only towards the end of the nineteenth century did the art of welding take a 
great surge forward with the introduction of the principle underlying Le 
Chatelier's blowpipe. The heat could now be taken to the parts to be joined. 

It is the oxy-acetylene flame that produces the heat required to bring 
about a fusion weld of two metal parts. This flame is the product of combus- 
tion of the two gases, oxygen and acetylene, supplied to the mixing chamber 
of the blowpipe or torch. The control of this flame is a matter of skill and 
experience, with manipulation of the oxygen and acetylene regulating 
knobs producing the following types of flame : 

(i) neutral flame (equal oxygen and acetylene) , 
(ii) oxidising flame (surplus oxygen), 

(iii) carburising flame (surplus acetylene) . 



© ® 



® 




Inner blue 

luminous 

cone 




Nozzle 



Feathery 
white plui 





Blowpipe 
nozzle 



Neutral Oxidising Carburising 

Fig. 1.5. — Welding Flames. 



These flames are simply illustrated in fig. 1 .5. The neutral flame is most used 
for welding; a maximum flame temperature of about 3000 C is possible. 
The size of the flame is determined by the bore of the removable nozzle at the 
end of the torch. A typical nozzle is shown at fig. 1.5D. 

Oxygen and acetylene are stored under pressure in separate bottles. These 
bottles are extremely strong solid drawn steel chambers. Each bottle is 
equipped with a head which carries a pressure regulator and two pressure 



PRINCIPLES AND APPLICATIONS OF WELDING 5 

gauges. The high reading pressure gauge indicates the pressure within the 
bottle; the second pressure gauge shows the pressure of gas fed to the torch, 
and this pressure may be adjusted with the regulator. 

Rigid safety precautions must be observed with regard to the storage and 
handling of both oxygen and acetylene bottles. Oxygen cylinders are usually 
painted black, with right-hand threads for the outlet connections. Acetylene 
cylinders are painted maroon, with left-hand threads on the outlet connec- 
tions. Oxygen and acetylene bottles must not be exposed to flames, heat or 
shock conditions, 

1.3. i The fusion process, or homogeneous welding 

Fig. 1 .6 illustrates the application of the oxy-acetylene flame in the weld- 
ing of the metal parts A and B. The weld shown is known as a vertical fillet 
weld. 

At the right hand we see the application of the welding blowpipe in heating 
the parts until a pool or puddle of liquid metal results. Note the use of filler or 
welding rod to augment or reinforce the joint. 

A section of the finished weld is shown on the left-hand side. On solidifica- 
tion of the liquid pool of metal, together with the additional metal supplied 
by the filler, a homogeneous joint results between the welded parts A and B. 



Filler rod 



Fillet 




Pool of molten metal 



Section through 
finished weld 

Fig. 1.6 



-Oxy- Acetylene Welding. 



With skill and experience the welder moves the flame and the filler rod 
along the joint at a predetermined rate, joining the parts with a pool of liquid 
metal augmented by the filler rod. Thin metal plates are leftward welded, 
that is to say the blowpipe flame points away from the deposited metal. 

Rightward welding is adopted for plates of thicker section. This means 



D WORKSHOP TECHNOLOGY I 

that the blowpipe flame points towards the deposited metal. Fig. 1.7 gives a 
typical application of both techniques. The great advantage of the oxy- 
acetylene welding process is considerable versatility in the type of welded 
joint possible, together with relatively easy manipulation of the blowpipe. 
There is, however, a very great deal of skill required by the user of the blow- 
pipe if efficient welds are to be produced. 



Circular motion 
of filter 
rod 



RK5HTWARP WELDING 



PATH OF BLOWPIPE 




Filler rod precedes 
flame 



Slight oscillation 
iOf blowpipe 



LEFTWARD WELDING 



Fig. 1.7. — Rightward and Leftward Welding. 

There is also the problem of holding and moving the filler rod in such a 
way that metal or joint augmentation is neither excessive nor insufficient. 

These factors militate against the use of oxy-acetylene apparatus for the 
automatic welding of metal parts, and we shall see later the great advant- 
ages offered by the arc welding process for the automatic or mechanical 
welding of metal parts on a mass or flow production basis. 

1.4 Oxy-acetylene flame cutting 

The oxy-acetylene flame can, with suitable modification, be used as a 
cutting tool on steel plates. This means that intricate profiles may be flame 
cut from steel sheet. The process is readily adaptable to mechanisation, with 
several highly efficient flame cutting machines available. 

Fig. 1.8 illustrates the profile of a component required in 50 mm thick 
mild-steel plate. If only one component is required, the profile is marked out 
on the steel plate. With a simple attachment known as a radius bar the 



PRINCIPLES AND APPLICATIONS OF WELDING 7 

circular portions are easily obtained, and simple devices are also used to cut 
the straight portions. In this way the profile of the component shown in fig. 
1.8 can be produced in reasonable time. 

In the event oflarge numbers of this component being required a profile 
cutting machine will be used. 



MILD STEEL COMPO NENTkSO- 



Cutting heads 
_^^"can be tilted 
for bevelled edges 




plate 
ALL DIMENSIONS IN mm 



Fig. i. 8.— Principle of Profile Cutting Machine. 

1.4. i Profile cutting machines 

These machines are used to produce steel components from flat stock. The 
cutting tool is an oxygen-enriched oxy-acetylene flame, capable of cutting 
through a steel plate more than 150 mm thick, leaving a remarkably good 
finish. 

All are copying machines ; it is necessary to make a template or drawing of 
the required profile, and many ingenious mechanisms are used to convert 
the outline of the component into movement of the cutting flame. 

The more modern profiling machines are capable of cutting accurate 
profiles on 150 mm thick steel plate by the electronic scanning of an inked 
drawing of the required part. Accuracies in the region of ± 0.1 mm are 
claimed for straight lines and gentle curves, while the electronic scanning 
head which guides or controls the path of the cutting flame cannot deviate 
more than 0.075 mm from the inked line. 

It is possible to have more than one cutting head on these machines, 
allowing very good production figures for complicated profiles in heavy 
steel plate. 

The principle of a flame cutting machine of the type described is shown in 

fig. 1.9. 



8 



WORKSHOP TECHNOLOGY I 



1.4.2 Principle of flame cutting 

Fig. 1.9A shows a section through the cutting head of an oxy-acetylene 
blowpipe or cutting torch. Note that the outlet of the cutting head has two 
orifices : the outer orifice supplies a mixture of oxygen and acetylene, and 
this flame is shown at fig. 1 .9B preheating the steel to be cut. 

When the steel has reached the correct preheating temperature of 900 C 
the operator depresses a small trigger on the welding torch. This releases a 
jet of pure oxygen through the centre orifice, which on striking the pre- 
heated steel brings about violent oxidisation. The pressure of the oxygen jet 
is such that the iron oxide and molten iron particles are carried away by tke 
pressure of the jet, and a neat cutting face results. '§. 

I 
Oxygen and 
acetylene 

Cutting oxygen 




Outer 
orifice 



SECTION ON X-X 

Fig. 1.9. — Technique of Flame Cutting. 

The cutting action is shown in fig. 1 .9C. For 50 mm steel plate the speed or 
rate of movement of the flame along the required profile is approximately 
12 m/h, or 3 mm/s. Thus a twin head profiling machine will produce two 
of the components shown in fig. 1 .8 in about five minutes. 

A clean-cut profile cannot be obtained on non-ferrous metals, and can be 
achieved only with difficulty on cast iron and stainless steel. This is owing to 
the fact that the oxides of these metals do not melt at a lower point than the 
metals themselves. In the case of steel, however, the melting point of the iron 
oxide, Fe 3 4 , is well below that of the iron or ferrite present in the steel. Thus 
the iron oxide melts and is blown away, leaving a relatively smooth edge. 
Non-ferrous metals melt at the edges, and an irregular, inaccurate edge 
results. 



PRINCIPLES AND APPLICATIONS OF WELDING 9 

1.5 Arc welding 

We have seen in our discussion on oxy-acetylene welding that additional 
metal is required to augment the molten pool in the path of the oxy-acetylene 
flame. The manipulation of the blowpipe and of the filler rod which provides 
the additional metal puts a severe limitation on the efficiency, and increases 
the cost, of the oxy-acetylene welding process. 

At the same time deep penetration of the metal requires large nozzles 
producing large flames with consequent risk of turbulence and scale within 
the weld. 

The great advantage possessed by the arc welding process is that the metal 
electrode used to strike the arc acts as a filler rod. This means that mechanical 
welding on a production basis is readily achieved. 

1.5. 1 Principle of arc welding 

The principle of arc welding is illustrated in fig. 1.10. This type of arc 
welding process is known as alternating-current arc welding. A trans- 
former is used, requiring little or no maintenance. 

High volts 

Low amps 

INPUT 



Electrode 




Primary 



OUTPUT 
High amps 
Low volts 



Metal 




Secondary 



Arc 

Fig. 1. 10. — Principle of Arc Welding. 



The purpose of the transformer is to change the high input voltage and low 
amperage to a low voltage and high amperage. The output from the trans- 
former is about 80 to 100 volts, and thus there is little risk of shock to the 
operator. The voltage required to strike the arc exceeds the voltage required 
to maintain it : an average striking voltage of 80 volts would result in a working 
voltage of 30 to 40 volts. 

1.5.2 The arc 

The arc is produced between the electrode tip and the parent metal. Fig. 
1 . 1 1 shows a close-up view of the arc produced from an a.c. transformer. The 



10 



WORKSHOP TECHNOLOGY I 



temperature of the arc can exceed 3000 C : this causes the formation of a 
liquid pool of metal and the transfer of metal from the electrode to the molten 
pool. 

Movement of the electrode in the direction of arrow A results in the deposi- 
tion of metal along the path of the electrode ; in this way a fusion weld is 
produced. 



Metal electrode 




Movement of electrode 



Electrode 



Coating 



Deposited 
metal 



Metal of 
electrode 
augments 
pool 

Liquid pool 



Parent metal 

Fig. 1.1 1— Arc Welding. 




Deposited metal 

Fig. 1 . 1 2 . — Shielded-arc Welding. 



It is necessary to maintain a constant gap between the electrode tip and the 
surface of the molten pool : this gap should be about the diameter of the 
electrode used. Fig. 1 . 1 1 shows the application of a bare metal electrode. It is 
clear from the diagram that the pool of liquid metal is open to the atmosphere, 
as also are the globules of molten metal leaving the metal electrode. 

1.6 Disadvantages of the bare metal electrode 

The welds produced by the process shown in fig. 1 . 1 1 are much weakened 
by the absorption of oxygen and nitrogen from the atmosphere into the weld. 
The absorption of these gases makes the weld brittle and porous, and this 
undesirable state of affairs is aggravated by the rapid cooling of the deposited 
metal. If the arc, electrode tip and molten pool can be protected or shielded 
from the atmosphere, the resultant weld will be much stronger. 

1. 6. 1 Shielded-arc welding 

This principle is illustrated in fig. 1.12. Note that the metal electrode is 
covered with a shield or coating. This coating is a type of flux, extruded under 
high pressure on the outside surface of the metal electrode, and possessing 
a higher melting point than the metal electrode. It will be seen in fig. 1 . 1 2 that 



PRINCIPLES AND APPLICATIONS OF WELDING II 

the coating extends beyond the electrode tip during the welding action. In 
this way further shielding against the atmosphere is afforded to the metal 
globules drawn into a liquid pool. Note not only that the melting of the 
coating produces a gaseous shield which acts as a barrier against the inclusion 
of both oxygen and nitrogen into the weld, but also that on solidification the 
melted coating forms an insulating layer of slag on the surface of the de- 
posited metal. This insulating layer promotes slow cooling of the weld while 
protecting the solidifying weld from the atmosphere. 

The use of a shielded-arc electrode also gives greater economy of welding 
rod, as vaporisation of the electrode is prevented by the shielding effect of 
the molten coating. Extensive use is made of shielded-arc electrodes in 
modern welding practice, and the automatic welding of steel parts is now 
commonplace. The development of the shielded-arc principle represents 
perhaps one of the greatest advances in welding technique. 

The welds produced are strong, reliable and ductile. The process is now 
widely used as a production tool in the fabrication of a great number of 
engineering components. 

Welding speeds are high ; the a.c. transformers are trouble-free and require 
little maintenance. The main limitation of this process is the length of the 
coated electrodes, which varies from 200 to 450 mm. 

Long runs require the changing of electrodes, with loss of welding time 
together with the risk of a discontinuous joint at the point of electrode chang- 
ing. Clearly if a coil of bare wire electrode could be used, automatic welding 
over long distances would be easily achieved. But a bare wire electrode, as 
we have seen, produces welds which have been exposed to the oxygen and 
nitrogen in the atmosphere, with serious weakening of the weld metal. 

If, now, a coil of bare wire used as an electrode produces an arc whilst 
submerged in a shielding flux, strong welds will result, equal in quality to 
those produced by the shielded-arc electrode. This is achieved in submerged- 
arc welding. 

1.6.2 Submerged-arc welding 

This technique is adopted for the production of long continuous welds, or 
the mass-production of welded joints. 

The principle is illustrated in fig. 1. 13. 

The flux is fed as a powder in front of the path of a moving head which 
carries the bare wire electrode coil. This flux has both high electrical resist- 
ance and heat insulating properties. The heat generated by the arc is confined 
to the weld area which is surrounded by the molten flux. In this way the flow 
of metal from the electrode is achieved in complete isolation from the harm- 
ful gases of the atmosphere ; the rate of cooling of the deposited metal is slow, 
and it is also protected from the atmosphere while cooling. 

At the end of the run the slag is easily broken away, and a neat, clean weld 
results. An automatic feeding device ensures that the gap between the elect- 
rode tip and the parent metal is constant. 

The welding process is entirely automatic and represents a tremendous 
advance in welding technique. The process is best used for the continuous 



12 



WORKSHOP TECHNOLOGY I 



welding of components having thicknesses from 12 to 50 mm. Typical appli- 
cations are the welding of pressure vessels and boilers. Considerable economy 
in weight is achieved if a boiler is welded instead of riveted ; the reduction in 
weight may be as much as 25 per cent, with the welded boiler capable of 
withstanding pressures of the order of 1 7 MN/m 2 . A further advantage of the 
submerged-arc process is that there is no flash or glare visible, the powdered 
flux completely shielding the arc. 

1.7 Argon arc welding 

This process is much used for the welding of stainless steel and non-ferrous 
metals such as aluminium, magnesium and copper. Two principles are in 
use, the first requiring the use of a filler rod to augment the metal of the weld. 
This is known as tungsten electrode welding. 



Coil of bare wire 
electrode with 
automatic feed 



•Moving head 



Flux supply 



Arc 




Fig. 1. 1 3. — Submerged-arc Welding. 

1.7. i Use of the tungsten electrode 

Fig. 1 . 1 4A illustrates a section through the working end of an argon arc 
welding torch. Note that the argon gas provides an inert shield against the 
atmosphere, allowing the use of bare metal filling rods. As no flux is used, no 
slag is formed and a clean weld results. 

1.7.2 Bare wire electrode 

Long continuous runs are achieved with the use of a wire electrode. This 
principle is shown in fig. 1.14B, and it will be seen that the technique is 
similar in many respects to that of submerged-arc welding. Reference to the 
diagram shows that the electrode, fed from a coil, augments the pool of liquid 
metal completely shielded by a curtain of inert argon gas. The whole unit is 



PRINCIPLES AND APPLICATIONS OF WELDING 



*3 



attached to a moving head which travels automatically along the path of the 
weld. The feeding of the wire is automatically controlled, with the arcing gap 
between the parent metal and the electrode wire tip at a constant distance. 
This technique is much used for the production of clean, slag-free welds in 
components fabricated from stainless steel and other non-ferrous metals. 

1.8 Pressure resistance welding 

All the welding techniques described so far result in fusion welds. No 
external pressure is applied. The principle in each case revolves around the 
formation of a liquid pool of metal between the two parts to be joined, with 
further augmentation of this homogeneous liquid pool using a filler rod or 
metal electrode. The continuation of this along the joint, either by hand or by 
automatic methods, welds the two parts together. 



© 



Water cooled 



Argon gas 



Filler rod 




Bare metal 
electrode 

Moving head 



To coil 



Fixed tungsten 
electrode 




Fig. 1.14. — Argon Arc Welding. 

There are, however, a great many occasions in engineering when welds are 
required to replace nuts and bolts or rivets in relatively thin metal. This need 
can be seen by referring back to fig. 1.2, where a fabricated pulley to take 
two V belts is shown. 

The front elevation shows that twenty-two spot welds are used ; these 
welds are indicated by lines aa on the sectional end elevation. The develop- 
ment of spot welding has been remarkable, and the process is now firmly 
established as being efficient and highly productive. 

1. 8. 1 Principle of spot welding 

The technique of spot welding is best applied to the joining of two metals 
of similar composition and thickness. 
The principle is shown in fig. 1.15. 



H 



WORKSHOP TECHNOLOGY I 



1.8.2 The electrodes 

These must possess high thermal and electrical conductivity. The bottom 
electrode is fixed, whilst the top electrode moves down and applies pressure 
to the work in the order of 70 MN/m 2 over the electrode contact area. 

When the correct welding pressure is reached, an electronic control passes 
a high amperage current through the electrodes. Because the electrodes are 
made from high quality copper with a little cadmium to increase their useful 
life, the high amperage current flows with little resistance through the 
electrodes, but meets with considerable resistance at the inner contact faces 
of the parts to be joined. 



Top electrode moves 
awayi on completion 
» of weld 




Bottom electrode 
fixed 



(Aj current on 



b; current off 

Fig. 1. 1 5. — Principle of Spot Welding. 



A rapid temperature rise takes place at the interface, resulting in the 
formation of a small pool of liquid metal. 

At this point the current ceases to flow, while the pressure is maintained 
until the pool of liquid metal or weld solidifies. 

Note that the weld does not extend to the outer faces of the parts being 
joined. This is due to the fact that the greatest resistance to the current flow 
is at the interface of the metal parts, and that the current is stopped as soon 
as the metal melts, as shown in fig. 1 . 1 5B. 

1.8.3 Applications of spot welding 

Clearly any welding process capable of replacing nuts and bolts or rivets 
finds a wide application in engineering manufacture. If, also, spot welding 
machines are available both as fixed and portable types, then there is practic- 



PRINCIPLES AND APPLICATIONS OF WELDING 



*5 



ally no limit to the use of spot welds as a method of permanently joining metal 
parts up to 10 mm thick. 

Provided the position of the weld is such that the electrodes can be applied 
at the points required, spot welding is a cheap, rapid and efficient technique, 
each spot weld taking only a few seconds or less. 

To give an indication of the efficiency of the spot welding process, the 
underbody of a motor car made mainly from 07 mm and 0-9 mm mild steel 
has about 1000 spot welds, yet the output rates are approximately 25 under- 
bodies per hour, or approximately one every two minutes. 

This output, of course, is achieved with the aid of special jigs, in which 
banks of electrodes move forward at predetermined intervals, the whole 
sequence being incorporated in an automatic press welding circuit. 

1.8.4 Seam welding 

Seam welding is adopted when watertight or airtight seams are required 
It is achieved by making a series of spot welds in such a way that the welds 
tend to run into each other or overlap. The principle is shown in fig. 1.16; 
note the use of circular electrodes. The application of the welding current is 
intermittent and is controlled by synchronous timing devices. The pressures 
exerted by the revolving electrodes are identical with those exerted by the 
electrodes used in spot welding. 



~~ pressure on circular 
electrode 




Seams of 
overlapping 
spot welds 



Fig. 1. 1 6.— Principle of Seam and Stitch Welding. 

With the electrodes revolving, a series of spot welds is made, each weld 
slightly overlapping its neighbour. The process is continuous and ideal for 
the joining of overlapping metal parts which make up watertight or airtight 
containers. 

1.8.5 Stitch welding 

Circular electrodes may also be used to stitch weld metal parts. This 
process consists in making a series of welds at regular or constant pitch, as 
shown in fig. 1.16B. 



i6 



WORKSHOP TECHNOLOGY I 



The roofs of most modern motor cars are stitch welded to the bodies, a hand 
operated, balanced circular electrode welding device being used. The prin- 
ciple underlying this technique is illustrated in fig. i . 1 7. The use of wheel type 
electrodes permits very high welding production rates, and once again we 
see the application of a welding technique as a production tool. Stitch welding 
is best considered as a form of continuous spot welding, and may also be 
carried out using the single electrodes shown in fig. 1.15. 



Roof 



Circular electrodes 



Spot welds 
IO mm apart 




Follows car roof 



Portable equipment 



Fig. 1. 1 7. — Stitch Welding Motor Car Roof. 

1.8.6 Projection welding 

This technique is mainly employed for the joining of metal parts which 
could be spot welded only with difficulty. 

The principle is identical with that of spot welding, namely the making of 
small localised areas of welds at predetermined points. 

Fig. 1 .18 shows two mild-steel pressings that are to be projection welded, 
the finished assembly making up a brake shoe for a motor vehicle. Clearly the 
spot welding of these components presents a difficult problem ; part A is 
assembled lengthways to part B. 

The application of the projection-welding technique is achieved by pro- 
viding part A with several projections, as shown in fig. 1.18. The principle 
underlying the projection welding technique is also shown in fig. 1 .18. 

Note that pressure is exerted on part A, followed by application of the 
welding current. This leads, as in spot welding, to local heating at the inter- 
face areas between the projections on part A and the face of part B. The pro- 



PRINCIPLES AND APPLICATIONS OF WELDING 

v&a& izzzzzzx Both 2 mm thick 



17 




Welding points 




Part B projection welded to part A 

Fig. 1. 1 8.— Mild-steel Pressings for Brake Shoe. 

jections melt and fuse with part B, fusion taking place simultaneously at all 
the contact points. 

In this way several spot welds are made in one operation, and the process 
is readily mechanised, with automatic feeding, welding, and removal of the 
welded assembly. With both parts produced by press blanking tools, the 



i8 



WORKSHOP TECHNOLOGY I 



manufacture and welding of this brake shoe is entirely automatic, with the 
very high output figure of 20 brake shoes per minute. 

The projection-welding technique may be used, also to weld plugs and 
bushes to heavy steel plate, or to weld together two relatively solid steel parts. 
The techniques are shown in fig. 1 .19, and it may be appreciated that con- 
siderable saving in time and material can result with the use of this technique. 




Bush 



Annular weld 



Section through welded assembly 

Fig. 1. 19. — Projection Welding of Bushes. 



1.9 Butt and flash-butt welding 

1.9. 1 Butt welding 

Fig. 1 .3, which shows a heavy duty lathe tool, provides a good example of 
the butt welding technique. It will be seen that the high-speed-steel cutting 
end of the lathe tool is butt welded to a medium-carbon-steel shank, thus 
providing considerable economy of the more expensive high-speed steel. 

The principle is shown in fig. 1.20, and consists in bringing together the 
two ends to be joined, and passing the welding current through the joint. As 
soon as the joint reaches the required temperature, further pressure is applied 
while the welding current is cut off. This technique is restricted to fairly low 
areas of contact, not exceeding three square centimetres, and it is essential 
that a very good match exists between the two areas to be joined. 

1.9.2 Flash-butt welding 

This technique is used for the butt welding of large sections, and is the 
actual method employed for the joining of the two separate steels making up 
the lathe tool shown in fig. 1.3. 



PRINCIPLES AND APPLICATIONS OF WELDING 



19 



The principle of flash-butt welding is shown in fig. 1 .2 1 . The mating areas 
need not be a good match or surface fit. Three separate stages are involved. 

Stage 1 . Preheating 

This consists in switching on the welding current and bringing the two 
ends to be joined in contact. Both ends immediately undergo a temperature 
rise. Because of the poor surface fit the temperature rise will vary across the 
interfaces. To prevent local overheating, the two ends are drawn slightly 
apart at regular intervals until the interface area is at a uniform temperature. 




Good mating surface 



CURRENT ON 



T 



STEADY 

Pressure 



-Joint at welding temperature 

fJNCREASED 
PRESSURE 



Fixed CURRENT OFF 

Fig. 1.20. — Butt Welding. 

Stage 2. Flashing 

This takes place when the two areas are in close contact and at the correct 
temperature. 

Stage 3. Upsetting 

As soon as the flash point is reached a heavy pressure is applied, resulting 
in the welding or fusion of the joint. Note that upsetting of the joint takes 
place, as shown in fig. 1.21(3). 

The welds produced by the flash-butt welding technique are of high 
quality, and the technique is readily applied to aluminium alloys and copper. 
When thinner sections, such as the rims of bicycle wheels, require to be flash- 
butt welded, a slightly different technique is employed. The two ends are first 
brought together under medium pressure, whereupon local heating takes 
place. At this point, owing to the uneven matching faces, the heating is not 



20 



WORKSHOP TECHNOLOGY I 



uniform and some arcing takes place. Continuation of the pressure causes the 
forcing out of the surplus or irregular metal at the interface, until finally the 
two ends are brought firmly together with the welding current switched off. 
The flash-butt welding of bicycle and other vehicle rims is invariably per- 
formed with the whole operation completely mechanised, and once again 
high output figures are easily obtained. 



Poor joint -> CURRENT ON 

-0:1./"/ 



Intermittent contact 

PREHEATING 



© 




FLASHING 



Heavy pressure 

UPSETTING 
® 



Upset 



CURRENT OFF 

Fig. 1.2 1. — Stages in Flash-butt Welding. 



i.io Importance of electrical resistance welding 

The resistance offered by a metal to the passage of a heavy current is the 
basis of all electrical resistance welding. 

Unlike electric arc or oxy-acetylene welding, where the fusion of the metal 
is achieved by the heat of the arc or flame, resistance welding employs only an 
electric current as the heat source. There is a complete absence of flash and 
glare, allowing the use of spot welding machines as part of a production line. 
With the use of suitable electronic equipment, the welding current time can 
be varied fromy^tf second for the spot welding of thin metal plate to 2 minutes 
for the flash-butt welding of heavy sections. 

Perhaps the most important feature of resistance welding is that the pro- 
cess is readily automated with complete elimination of the human element. 



1. 11 



Testing of welds 

The testing of welded joints differs little in principle from the standard 
tests adopted to determine the mechanical properties of the parent metals 
from which the joints are made. 

For example, the tensile strength, resistance to impact and ductility of a 



PRINCIPLES AND APPLICATIONS OF WELDING 21 

welded joint can be readily determined from a specimen weld, using stand- 
ard testing machines. 

It must be clearly understood, however, that the results obtained refer 
only to the test piece under test, and if a large welded structure, say a pressure 
vessel, has been hand welded, it is possible that faults may exist as a result of 
the quality of the workmanship. 

Clearly it is not practicable to cut away a section of the pressure vessel in 
order to determine the efficiency of the welded joint, and this means that all 
weld testing techniques must fall into one of the following two categories : 
(i) destructive tests 

(ii) non-destructive tests. 

1. 1 2 Destructive tests 

In addition to the standard tests for tensile strength using a tensile testing 
machine, or resistance to impact using an Izod impact testing machine, 
much useful information can be gained by carrying out a macrostructure 
examination of a section of the welded joint. 

1.12.1 Macrostructure examination 

The following remarks give a brief indication of the procedure adopted 
when making a macrostructure examination of a welded joint ; books on 
materials will supply more detailed information. 

The surface area of the weld section under test is brought to a smooth 
polished condition. An etching reagent is applied ; for mild steel this is usually 
a solution of nitric acid and alcohol. The effect of the reagent is to etch away 
the grain boundaries of the welded area, thus not only showing the crystal 
size, but also revealing the presence of discontinuities and cracks. 

1. 13 Non-destructive tests 

Apart from close visual examination of the weld, from which an experi- 
enced inspector is able to make a fair assessment of the weld quality, there 
are several non-destructive tests in use. 

A popular test for the magnetic metals, such as the carbon and certain 
alloy steels, is the magnetic method of crack detection. 

x.13.1 Magnetic crack detection 

This method is used as a supplement to visual examination. It is not 
possible for the human eye to detect, unaided, minute or very fine hair cracks 
which may be present in or around the surface of the weld area. But provided 
the weld area under test has a reasonably smooth surface it is possible to 
detect even the finest hair crack when using the magnetic crack detection 
method. 

Briefly the principle consists in passing a magnetic flux through the metal 
of the weld, after covering the surface with a mixture of magnetic iron oxide 
powder and paraffin. The presence of any break or discontinuity on the sur- 
face of the metal creates a magnetic field across the break or crack, and the 
magnetic iron powder is attracted in considerable quantity to this field. 



22 



WORKSHOP TECHNOLOGY I 



The principle is illustrated in fig. 1.22. Note that the powder is attracted 
along the direction of the crack, thus providing clear visual evidence of the 
presence of a break or discontinuity of the metal. 



-I- Pole of 
electro- 
magnet 



Magnetic field 



Hair crack 




Small hair 
crack 



-Pole of 
electro magnet 



Magnetic powder 
accumulates along 
crack 



Enlarged 
view on 
section Y-Y 



Fig. 1.22.— Principle of Magnetic Crack Detection. 

1.13.2 X-ray examination 

This technique is widely used for the testing of welds in pressure vessels 
and similar structures. It is essentially a non-destructive testing technique. 
Although the apparatus is relatively expensive, it permits an assessment to 
be made of the quality of the weld with regard to internal defects. 

The joint is exposed to a beam of X-rays, and the amount of this radiation 
absorbed by the metal depends on the amount of metal opposing the path 
of jthe X-rays. 

The presence of any internal voids, cavities or defects presents no opposi- 
tion to the X-ray radiation, and a photographic plate or film can be used to 
reveal variations in the path of the X-ray radiation. There are also many 
other non-destructive testing techniques in use; much skill, patience and 
experience is required for their efficient use. 

Summary 

Workshop technology may be described as the practical application of the 
scientific principles underlying manufacturing techniques or processes. 
There is little doubt that of all the manufacturing processes used in modern 
engineering manufacture, the welding process demands the greatest know- 
ledge of scientific principles. 

If we consider also the fact that the greatest advances in recent years in the 
field of production techniques have been made in the art of welding, then it 



PRINCIPLES AND APPLICATIONS OF WELDING 23 

is clear that welding now represents a major activity in modern production 
engineering. 

It is not so long ago that welding was considered primarily as a process for 
the repair or renewal of broken components, but we have seen in the pre- 
ceding pages that welding is now an accomplished and proven production 
engineering technique by which metals are joined as part of a mass-produc- 
tion process. 

Although the principle underlying the joining of two metals remains con- 
stant, namely the formation of a liquid metal pool between the interfaces of 
the parts to be joined, several sources of heat supply are adopted. 

The use of the oxygen and acetylene flame is essentially a chemical process, 
requiring considerable care in the storage and pressure of these gases, to- 
gether with the need for protection of the eyes of the welding torch operator. 
There is need, too, for the use of a filler rod as a means of augmenting the weld 
by the addition of extra metal. This makes the production of long continuous 
welds of uniform strength a difficult matter, and the exposure of the weld to 
the oxygen and nitrogen of the atmosphere reduces the efficiency or strength 
of the weld. 

The shielded-arc principle produces much stronger welds, free from the 
weakening influences caused by the absorption of the atmosphere into the 
weld, while the arc produced between the work and the electrode used as a 
filler rod provides a clean and troublefree source of heat. Because the length 
of the flux-covered electrode limits the amount of welding possible as a 
continuous run, much use is made of a coil of bare wire electrode, with 
shielding from the atmosphere achieved by surrounding the weld with 
powdered flux (submerged-arc welding), or providing a shield of inert gas 
(argon arc welding) . 

These techniques allow not only the automatic welding of large-section 
ferrous welds, but also automatic welding of thin-section non-ferrous metals. 

Large sections can be butt or flash-butt welded, a heavy electrical current 
providing the heating source. For the localised welding of relatively thin steel 
components, great use is made of electrical resistance pressure welding, 
typical techniques comprising spot, seam and stitch welding. 

The types and varieties of welding machines available to the engineering 
industry are very large in number. Most of the welding machines using elec- 
tricity as a source of heat are equipped with complicated electronic devices 
controlling not only the amount of the welding current, but also the time of 
current application. 

These machines, rightly, are the province of the electrical engineering 
technician, but it is not a bad thing if the mechanical engineering technician 
appreciates some of the basic principles underlying their design and appli- 
cation. 

EXERCISE 1 

1 Show by means of a simple line diagram the techniques adopted for the production of 
fusion welds and pressure welds. What is the essential difference between a fusion weld 
and a pressure weld? 



24 WORKSHOP TECHNOLOGY I 

2 (a) With a neat diagram illustrate the technique of oxy-acetylene welding. 

(b) What is the purpose of the filler rod? 

(c) Give a typical application for oxy-acetylene welding. 

3 (a) What is meant by the term "flame cutting" ? 

(b) Why is this process generally restricted to ferrous metals such as the carbon steels? 

(c) Make a neat sketch of a component that could have its profile flame cut. Describe 
briefly the technique adopted if a large number of similar components is required. 

4 (a) What is the essential difference between arc welding and oxy-acetylene welding? 

(b) Make a neat sketch illustrating the principle of arc welding. 

(c) Describe the manner by which additional metal is added to an arc weld. 

5 Describe a typical application for the technique of submerged-arc welding. List three 
advantages of this process. 

6 What is the purpose of a covered or shielded electrode as used in arc welding? Explain 
the limitations of covered electrodes that are to be used on a production long-run welding 
operation. 

7 (a) Describe two techniques adopted to prevent atmospheric contamination of the 
weld when using bare wire electrodes. 

(b) Give a typical application for each technique. 

8 (a) Show by means of a neat sketch the welding principle adopted to replace rivets 
when joining thin mild-steel components. 

(b) Sketch a typical application of the use of spot welding when joining mild steel 
pressings to make up an engineering assembly. 

9 (a) Sketch typical applications of the following welding techniques : 

(i) seam welding 
(ii) stitch welding 
(iii) projection welding 
(iv) butt welding 
(b) Describe one of the above welding principles in some detail. 
io Describe in some detail an application of each of the following testing techniques : 
(i) non-destructive testing 
(ii) destructive testing. 



Measurement 



2.1 The need for precision measurement 

The mass-production which characterises so many branches of modern 
engineering manufacture would be impossible if component parts could not 
be produced to close dimensional tolerances and thus made interchange- 
able. Motor vehicles, refrigerators and washing machines, to name only a 
few examples, could not be made available in the quantities our modern 
civilisation has come to take for granted. 

It is seldom, however, that the components themselves are subjected to 
precision checks, and it is therefore essential that the accuracy required 
should be built into the machine tools, jigs and fixtures which produce them. 
Precision measurement is concerned with the precise determination of the 
linear, angular and non-linear functions of the machined surfaces of the 
tools and devices used to produce engineering components. 

Fig. 2.1 illustrates two pressed-steel components used to make the brake- 
shoe previously described and illustrated in fig. i . 18. The parts are produced 
with press tools at the rate of about twenty per minute. Note that part B has 




Projections 




X Location slots X J 

Fig. 2.1. — Mild -steel Pressings for Brake Shoe. 



25 



26 WORKSHOP TECHNOLOGY I 

ten pierced holes to accommodate the rivets holding the brake lining; note 
also the location slots and the raised projections shown as Y. 

The dotted lines on component A represent further piercing and notching 
operations carried out after projection welding part A to part B. Ifwe consider 
the manufacturing time for this component, namely three seconds, then 
clearly it is an impossible task to measure the linear dimensions of the hole 
centres, slot and projection positions. We must rely on the part being a faith- 
ful replica of the tools used to produce it, and for this reason it is vital that the 
dies and punches are both made and measured as accurately as possible, 
and within the specified limits laid down in the drawing or blueprint. 

Precision measurements must be carried out on both the dies and the 
punches of the press tools used, and provided the dimensions are within the 
limits laid down the press tool can be put into production with every confi- 
dence in the acceptability of the parts produced. We see now that precision 
measurements are always required in the manufacture of press tools, and 
much the same can be said for the manufacture of machine tools such as 
lathes, milling machines and drilling machines. The dimensional and geo- 
metrical accuracy of the components produced using the above machine 
tools is proportional to the inherent accuracy built into the machine tool, 
and thus the operator or craftsman is able to produce accurate work. 

2.2 Limitations of line standards 

Equipment for the direct measurement of linear dimensions falls into two 
categories : line standards and end standards. Engineers' steel rules, vernier 
calipers and vernier height gauges are all examples of line standards ; they are 
described in detail in Workshop Processes i, Chapter 4, and 2, Chapter 5, 
where it is pointed out that the accuracy of line standards is limited by the 
fact that the engraved lines themselves possess thickness. Greater accuracy 
can be achieved with the use of end standards. 

2.3 End standards 

2.3.1 Slip gauges 

Some typical applications of those end standards which are more com- 
monly known as slip gauges may be found in Workshop Processes 2, Chapter 5. 
It is seldom, however, that slip gauges are used to determine linear dimen- 
sions in excess of 250 mm, and this is for the following reason. To build up a 
measurement of 250 mm, assuming that an 83 piece slip-gauge set is avail- 
able, four slips are required (100, 90, 10 and 50 mm). This means that the 
overall height of the slip pile will be slightly in excess of 250 mm, owing to the 
accumulation of the plus tolerances on each slip. More accurate determina- 
tion of linear distances in excess of 250 mm is achieved with the use of end 
measuring bars. 

2.3.2 End measuring bars 

Fig. 2.2. shows a section through a large blanking die; it is required to 
check the distance between two die inserts. This distance, 480-25 mm, is 
checked using end measuring bars in conjunction with slip gauges. Manu- 



MEASUREMENT 27 

factured from high grade carbon steel, hardened at each end and specially 
processed to give perfect stability, end measuring bars are about 20 mm in 
diameter, and a typical set will have the following bars : 

25> 5°> 75> 100, 125, 175, 200, 375, 575, and 775 mm. 

This set will give combinations up to 975 mm with two bars, and combina- 
tions up to 1550 mm with three bars. 

Workshop bars are provided with tapped holes at both ends, allowing the 
bars to be joined together without any appreciable loss of accuracy, and a 



480-25 



Hardened 
~| I die insert 




I OO 



375 



Fig. 2.2. — Measuring the Width of a Blanking Die. 



useful range of accessories is available. Both bars and accessories are manu- 
factured to very close limits of accuracy, for example the tolerance at 20 C 
on a 1 00 mm workshop bar is plus 00005 mm, whilst the Inspection, Calibra- 
tion and Reference sets are made to even greater degrees of accuracy. 

Fig. 2.2 shows the measuring technique involved in the use of end measur- 
ing bars. The overall distance of 48025 mm is made up byjoining the 375 mm 
bar to the 100 mm bar, as shown in fig. 2.2B, whilst workshop slip gauges of 
4 mm and 1-25 mm are further wrung to one end of the measuring bars. 
Provided great care is used, together with a nice sense of touch or feel, the 
user will be able to determine the width or distance between the die inserts 
to within plus and minus two micrometres. 



28 



WORKSHOP TECHNOLOGY I 



Support of end measuring bars 

Fig. 2.3 shows, in exaggerated form, the conditions caused by incorrect 
support of end measuring bars. At A we see the bars supported at each end, 
giving rise to a sag or droop at the centre ; whilst at B, with the bars supported 
at mid-point, drooping of both ends takes place. 

Clearly the conditions at both A and B give rise to an out-of-parallel con- 
dition of the ends of the measuring bars, and as these are the measuring faces, 
errors must result. The correct supporting technique is shown in fig. 2.3C. 
Note that the bars are supported at what are known as the Airy points. As 
shown in the diagram, the distance between these points is equal to the length 
of the composite bar divided by the square root of three. End measuring 
bars supported in this way will have minimum deflection or bending. 




Sag at aentre 



Sag at ends 




® 




® 



t- 



AIRY POINTS I 

/l. \ : 



Fig. 2.3. — Airy Points. 

2.3.3 Standard or precision rollers and balls 

Sets of precision rollers are available ranging from 1 mm to 25 mm dia- 
meter. These rollers are made from good-quality steel, hardened and tem- 
pered, with the length of each roller equal to its diameter. Both diameter and 
length are within two micrometres of the stated size. Precision steel balls 
are also available in sets, and the use of rollers and balls in conjunction with 
slip gauges and end measuring bars permits a wide range of methods when 
determining linear dimensions to a high degree of accuracy. 

The following examples will serve to illustrate the principles and tech- 
niques involved in the application of end standards to determine both linear 
and angular dimensions. 



MEASUREMENT 



29 



EXAMPLES 

(i) Fig. 2 .4 shows a plan view of a die for a press tool. It is required to check 
the radius shown as R. We may assume that the die has been brought to the 
inspection department for checking, and we are now concerned only with 
the determination of the linear distance or radius R. 



r\ 




55 


O 


\J 






6>*- 


*5 

30 *R 


O 


O 






^ 


"1 
>5 


O 






O 




so 







All dims in mm 



All tolerances t 002 

Fig. 2.4. — Die for Press Tool. 



The set-up is illustrated in fig. 2 5A. Note that a 50 mm slip gauge is placed 
in the part circle as shown, with a 5 mm roller placed at the bottom of the part 
circle. The problem now is to calculate the slip-gauge height that will just 
enter the gap between the top surface of the roller and the bottom surface of 
the 50 mm slip gauge. At fig. 2.5B we see the essential geometry involved; 
referring to the diagram : 

Let height of slip gauges = H 
then//= AD-(AC + d) 
or H=R-{AC + d) 

The problem now is to calculate the distance AC ; this may be carried out as 
follows : 

R 2 = BC 2 + AC 2 
'• AC 2 = R 2 -BC 2 
:. AC = Jftf-BC 2 ) 

— V 2 75 = 1658 mm 



30 



WORKSHOP TECHNOLOGY I 



Now distance H = R— (i6-$8 + d) 
= 30-( l6 *5 8 + 5) 
= 8-42 mm 

This means that a slip gauge pile of 842 mm should just enter the gap be- 
tween the roller and the 50 mm slip gauge. An experienced inspector will be 
able to determine the actual slip height that just enters, and thus determine 
the error by rearranging the formula used to obtain the theoretical height of 
slips. 

(ii) The principle of the dovetail slide is widely used in machine-tool con- 
struction, and the measurement of both the linear and angular dimensions 
of these dovetail slides provides interesting examples, not only of the use of 
precision balls and rollers, but also of the need for a good working knowledge 
of geometrical and trigonometrical principles. 



All dimensions in mm 




^ ^~— Diameter of roller' 

Fig. 2.5. — Determining an Internal Radius. 

Fig. 2.6A shows an external dovetail; it is required to check the distance 
X, which is the width of opening. At fig. 2.6B we see the set-up for the job. 
Two rollers of equal diameter are placed in opposite corners as shown. By 
trial and error the distance shown as H in the diagram is obtained with the 
use of slip gauges. If this distance is in excess of 250 mm, then end measuring 
bars must be used. 

Distance ^may be calculated from the following formula: 

X = h + d+ldcot* 

where h = slip-gauge distance, 
d = diameter of rollers, 
a = angle of dovetail. 



MEASUREMENT 



3* 



Internal dovetails may also be checked in like manner with the aid of slip 
gauges and precision rollers. Fig. 2.6C shows the technique adopted. Note 
the use of a slip-gauge nest or cage, in which slip gauges are clamped, thus 
permitting easier manipulation of the assembled slips, and also avoiding 
handling of slip gauges during the measuring operation (fig. 2.6D). 

An external micrometer may be used to determine the distance h, and once 
again the accuracy achieved will depend on the experience and skill of the 



user. 



External dovetail 




Cage^ 



"Roller of 
diameter- d 



Internal 
dovetail 



a 



®TE 



n 



Clamping screw 



^SMp gauges 

Fig. 2.6. — Measuring Internal and External Dovetail Slides. 
The width of the opening shown as ^f in fig. 2.6C may be calculated from 
the following formula : 

X = h-d- Id cot* 

(iii) It has been assumed in Examples (i) and (ii) that the angles of the 
dovetails are within the limits laid down in the drawing or blueprint. It is not 
difficult, however, to check these angles, and fig. 2.7 shows the technique 
adopted. 

Two rollers of identical size are placed as shown, and the distance across 
their outside faces determined. The same rollers are now placed on equal- 
height slip gauges, as shown in fig. 2.7, and once again the distance across 
their outside faces is determined. 



If 

and 

then 



Y — the first linear distance, 
X = the second linear distance, 
h 
tana= (T-X) 



32 




WORKSHOP TECHNOLOGY I 



Slip gauges of equal height 




I st Position of rollers 
Position of rollers 

Fig. 2.7. — Angular Check of Dovetail Slide. 

The determination of the angle of an internal taper provides a good example 
of the use of precision balls. 

(iv) Fig. 2.8 shows a milling cutter adapter; it is required to check the 
internal taper. This is achieved by carefully inserting, in turn, two precision 
balls as shown in the diagram. With the smaller-diameter ball in position 
close to the small end of the taper, the adapter is placed on a toolmaker's flat, 
with two slip-gauge piles of equal height placed either side. 

Depth micrometer 




Grade A surface plate 
(Toolmaker's flat) 

Fig. 2.8. — Determination of an Internal Taper. 



\ 



MEASUREMENT 



33 



A depth micrometer is now used to determine the distance from the top 
surface of the slips to the surface of the precision ball. The same procedure 
is repeated when the small ball is removed and the larger ball rolled into 
position. The distances shown as H x and H 2 on the diagram may now be 
calculated and the angle of the taper found from the formula 

. a (R-r) 
sin - = v w ' 
2 EF 

where a = included angle of taper, 
R = radius of large ball, 
r = radius of small ball, 
EF = centre distance of balls. 

The examples described represent only a few of the very many applications 
of working standards to determine linear and angular dimensions to close 
limits of accuracy. It is essential that all the equipment be carefully checked 
before use. For example, the depth gauge used in Example (iv) should be 
tested using slip gauges, with a toolmakers' flat used as a reference plane or 
datum surface. The method is shown in fig. 2.9; note the adjusting screw, 
which will permit the depth micrometer to be adjusted should an error be 
detected. 



Adjust nut 
to give zero 
reading 



* 25- SO Rod 





All dimensions in mm 



£ 



20 and 5 
Slip gauges 



V////////J 

Grade A reference plate ' 

Fig. 2.9. — Checking Accuracy of Depth Micrometer for Zero Setting. 

2.4 Limitations of direct measurement 

Perhaps the best way of demonstrating the limitations of direct measure- 
ment using working standards is for the student to carry out for himself the 
measuring techniques described in the four examples given. Better still, 



34 



WORKSHOP TECHNOLOGY I 



let (say) four students each carry out the determination of the respective 
dimensions independently, and then compare results. It is certain that there 
will be discrepancies, and these will not all be caused by arithmetical errors. 
In all the examples given, the troublesome question of feel exists. With pract- 
ice it is possible to find the height of slip gauges that will just enter a gap, with- 
in two micrometres from a height which will not enter or just nip. This 
means that the distance H shown in fig. 2.10 can be determined to within 
approximately one micrometre or o-ooi mm. Fig. 2.10 shows also the use 
of a toolmaker's straight-edge to determine the overall height of the com- 
ponent together with the parallelism of the top face with the base. Clearly 
much depends on the skill of the inspector, and to measure large numbers of 



All dimensions in mm 

if 112-75 enters 
and 112*755 nips 

H=M2-752 5 (app) 



Slip 
gauges 



Toolmakers 
straight edge 




Grade A reference plane 

Fig. 2.10. — Use of Slip Gauges and Toolmaker's Straight-edge. 

components using the equipment and techniques under discussion will 
present very great difficulties, owing to the variations that must result as a 
direct outcome of the human element. It must now be appreciated that if 
precision measurements are to be made with consistent accuracy, then as 
far as possible the problem of feel must be solved, and this can only be achieved 
with complete removal of the human element. 

At the same time we need to magnify the difference between the linear 
dimension under test and the working standard used. If this difference can be 
clearly shown on a dial or any other recording device, then clearly it is possible 
for precision measurements to be made reasonably free from error intro- 
duced by the inspector. This is the principle of measurement by com- 
parison. 



MEASUREMENT 



35 



2.5 The use of comparators 

Measurement by comparison can be made only with the aid of a suitable 
comparator. The function of any comparator is to magnify and record any 
difference in height between the working standards used to set the com- 
parator to zero and the component under test. Let us assume that we have a 
large number of precision rollers of 40 mm diameter, and that we wish to 
measure the diameter of each roller. 

It is not as we have seen, a practicable proposition to employ (say) six in- 
spectors, giving each a 25-50 mm micrometer. This must entail individual 
measurement of each roller, with any variation due to the human element 
affecting the resulting measurements. Clearly a better plan is to select an 
an end standard of 40 mm, and then compare the height of each roller against 
this known standard. We may merely select the 40 mm slip gauge from the 
Inspection set, knowing that this slip gauge is accurate to within two tenths 
of a micrometre. With this slip gauge wrung to a toolmaker's flat, which is 
equivalent to a high grade reference plane, all that is now needed is a precision 
comparator that will clearly magnify any linear displacement of the measur- 
ing plunger, as shown in fig. 2 . 1 1 . 



Each division* 0*002 




Difference 
magnified and 
shown on 
dial 



Each division-0002 
All reodings in mm 



Fig. 2.1 1. — Principle of Measurement by Comparison. 



Note the indicating dials shown as A, B and C. The magnification of the 
comparator used is the ratio between the distance moved by the pointer on 
the indicator scale and the distance moved by the plunger. At A we see the 
popular 0.02 mm comparator, more commonly known as a dial test in- 
dicator. The width of one division on the dial of this type of instrument is 



36 WORKSHOP TECHNOLOGY I 

about i 5 mm ; thus the magnification is approximately 75 : 1 . It is unlikely 
that this type of comparator would be used to check the height of the rollers, 
because it is necessary to calculate or estimate to within one micrometre. 

At B we see the dial of a 0002 mm dial indicator. Each division is now 
equivalent to a plunger movement of two micrometres, and 0.02 mm move- 
ment of the plunger results in pointer movement of approximately 10 mm. 
Thus the magnification of this instrument is of the order of 500 : 1 . 

At C we see the dial of a comparator most likely to be used to determine the 
diameters of the rollers under test. Note that the measurable range is only 
plus and minus two hundredths of a millimetre. This means that this com- 
parator cannot be used to check components having tolerances in excess of 
plus and minus two hundredths of a millimetre. A movement of 002 mm of 
the plunger gives rise to a pointer movement of about 20 mm; thus the in- 
strument has a magnification of 1 000 : 1 . 

Clearly it is a relatively simple matter to set this instrument to read zero 
with the plunger resting on a 40 mm inspection slip gauge, as shown at D. 
Any variation in height is immediately indicated by the pointer as the rollers 
under test are passed beneath the plunger. Although a division on the dial 
is about two millimetres in width, representing a plunger movement of 
0002 mm, it is not difficult to estimate the pointer position to within one 
quarter of a division. In this way roller diameter errors may be detected to 
within half a micrometre. 

This technique solves several of our measuring problems. Firstly, as we 
have seen, considerable accuracy is achieved, according to the type of com- 
parator used. Secondly, not only are we able to use end standards, but also 
we are able to do so with the troublesome aspect of feel removed. We may 
define feel as the pressure exerted on the slip-gauge surface by the plunger 
when setting for zero. If the precise comparison is to be made, it is essential 
that the plunger exerts the same pressure on the roller under test. This of 
course is exactly what the instrument is capable of doing, irrespective of the 
skill and experience of the person who passes the roller under the plunger. 

Thirdly, then, the rollers can be checked using relatively unskilled operat- 
ors, and the fourth and perhaps most important point is that the measuring 
operation is performed in a very short space of time. 

The reasons given above account for the wide and increasing use of the 
principle of measurement by comparison in the measurement of engineer- 
ing components. 

2.6 Types of comparators 

Several principles are adopted in order to obtain magnification of plunger 
movement, and a comparator used for the measurement of engineering com- 
ponents may be classified under the following headings : 

(i) mechanical, 

(ii) electrical, 
(iii) optical, 
(iv) pneumatic. 



MEASUREMENT 37 

Irrespective of the principle used, all comparators should satisfy the 
following essential requirements : 
(i) The comparator should be able to stand up to normal wear and tear 

without loss of accuracy. 
(ii) The measuring head should be capable of vertical adjustment to allow 

for the checking of components of differing heights. 

(iii) Means should be provided to allow the instrument to be set to zero. 

(iv) The reading should be dead beat. This means that the pointer is free 

from oscillations, and responds immediately and positively to the 

plunger movement. 

(v) A hard-wearing contact should be provided at the working face of the 

plunger, and the force exerted by this point or contact face should not 

exceed 3 N. A value of about 25 N is acceptable. A simple lever or 

other similar device should be available for easy lifting of the plunger. 

2.7 Mechanical comparators 

As the name suggests, a mechanical comparator uses mechanical prin- 
ciples in order to obtain magnification of the plunger movement. The 
principles involved are those used by engineers when transmitting motion 
from one part of a mechanism to another. If a mechanical comparator has 
a rotary dial, as shown in fig. 2 . 1 1 A, B and C, then clearly a simple definition 
of the requirements is maximum rotary movement of the pointer for mini- 
mum linear movement of the plunger. The only disadvantage is that any 
wear, play, backlash or dimensional faults in the mechanical devices used 
will also be magnified. 

2.7.1 Dial indicators 

Essentially a dial indicator is a mechanical comparator using gear systems 
together with a rack and pinion. A suitable spring gives constant plunger 
pressure, whilst hair springs may be employed to eliminate play or backlash. 
If a dial indicator is to provide faithful magnifications of the plunger move- 
ment, the dimensional and functional features of the gears, racks and pinions 
used must possess a high degree of precision. Dial indicators, however, seldom 
exceed 60 mm in diameter, and this means that the moving parts are of neces- 
sity quite small. This fact increases the difficulty of machining these parts 
to the very high degree of precision required ; thus dial indicators are limited 
to an accuracy of about 0-002 mm. 

It will be found that dial indicators capable of reading to 0002 mm require 
very great care in use and handling ; very often better results are possible using 
a dial indicator with graduations of 001 mm, or even 002 mm. 

2.7.2 Correct use of a dial indicator as a comparator 

If a dial indicator is to be used as a comparator, the set-up shown in fig. 
2.12 should be adopted. Note the rigid column, with provision for vertical 
adjustment, and the small accurate reference plane, or worktable, with pro- 
vision for fine adjustment. Such a simple comparator is ideal for the checking 
of components to within a tolerance of, say, plus and minus 0-05 millimetres. 



38 WORKSHOP TECHNOLOGY I 

Note, too, the use of adjustable limit indexes; it is now a simple matter to 
determine whether large numbers of components are machined to within 
the tolerance of plus and minus 0-05 mm. With the comparator set to middle 
limit using slip gauges, and the limit indexes set 0-05 mm each side of the 
zero position, rapid and efficient measurement of the components is readily 
achieved by unskilled operators. Clearly, if the operator is instructed only to 
reject those components that cause the pointer to record outside the limit 
indexes, the comparator is now used as a visual gauging device. It is not 
strictly necessary for the operator to be made aware of the fact that each 
division on the dial of the dial indicator represents 002 mm movement of the 
plunger. The operator of the comparator is now, in effect, gauging the dimen- 
sion under test ; that is to say, merely ensuring that the dimension is within 
its high and low limit and thus acceptable. 



Column. 




Limit indexes 



indicator 

height 

lock 



TT,.^ Plunger 

/Work 
/table 



Table 





Fig. 2.12. — Dial Indicator Comparator. 

2.7.3 Lever comparators 

The principle of the lever when used to obtain magnification of small 
linear movements is appreciated by all toolroom turners, who often make use 
of the device shown in fig. 2.13. This simple device, usually referred to as a 
wiggler, is used to assist the turner in centring, say, a centre dot or drilled 
hole prior to further machining. The illustration at A shows the working 
point inserted in a centre dot ; if a drilled hole requires centring, a small steel 
ball is placed over the point. 

The geometry of this simple magnifying device is shown in fig. 2 . 1 3B. If 
the centre dot does not lie on the lathe centre line, linear displacement of the 



MEASUREMENT 39 

working point of the rod takes place; thus a movement of the working point 
shown as AC gives rise to a movement at the recording end of the rod EB. 
Note that the magnification is of the order of i o : i . Note also that the distance 
AC is a linear distance at 90 to CE, and is equal to r sin 6, whilst the magnified 
movement at the recording point is along the arc EB. This distance is equal to 
Rd, with the angle 6 in radians. This magnification of straight-line displace- 
ment in the form of movement along an arc sets a limitation on the use of this 
principle with regard to its adaptation for comparators. This is because the 
scale representing the magnified movements of the working point or plunger 
will have unequal divisions. In other words, unless the angular movement 
is very small, equal increments of the plunger do not produce equal incre- 
ments of the angular movement of the pointer. Fig. 2.14 illustrates a simple 
mechanical comparator in which the lever principle is used in conjunction 
with a sector-and-pinion device. Note that movement of the plunger causes 
the sector arm to rotate slightly about the fulcrum O. Thus the point A moves 
along a small arc, as does the point B ; equal angular movements of the sector 
arm produce equal angular movements of the pointer. This means that the 
divisions on the scale are equal. 



& 



.Component held in chuck 

25 mm 250mm 




AC=rSirv6- EB= R-6- rads /g\ 

Fig. 2.13. — Principle of a Lever Comparator (Wiggler). ^-^ 

The scale for this instrument is also shown in fig. 2.14, and it will be seen 
that a range of 02 mm limits the plunger movement to this amount. Each 
division on the scale is equivalent to a plunger movement of 0005 mm. 
Perhaps one objectionable feature of the principle shown in fig. 2.14 is that 
the pointer records a magnification of upward movement of the plunger. 
This means that if the plunger is accidently subjected to a sudden upward 
blow (something very easily done when setting the instrument to zero) there 



40 WORKSHOP TECHNOLOGY I 

is a serious risk of damage to the delicate mechanism. Stops are always fitted 
within the instrument to guard against this, but better types of mechanical 
comparators are designed in such a way that the pointer is actuated only by 
downward movement of the plunger; thus accidentally knocking the 
plunger upwards has no ill effect on the delicate mechanism of the instrument. 

2.7.4 Twisted-strip comparator 

The principle underlying the design of this comparator has for many years 
formed the basis of a simple mechanical toy. It is hoped that all students will 
be able to recognise the device illustrated in fig. 2.15A. If we remember that 
the purpose of a comparator is to convert small linear movements of a plunger 
into large circular movements of a pointer, then clearly the twisted-strip 
principle has something to offer. 

-Scale 

02 mm ra nge 

y ^" u \n ^-— ~~ 

Pointer 





each small 

division « 0005 mm 



Plunger 

06 N gauging force 

Fig. 2.14. — Upward Reading Mechanical Lever Comparator. 

For very small linear movements of the twisted cord in the direction of the 
arrows, the disc rotates at considerable speed ; a point X on this disc would 
move through a very great distance indeed. This is the principle of the twisted- 
strip comparator, and the mechanism is indicated in fig. 2.15B. Vertical 
movements of the plunger are transferred to the right-hand side of a thin 
twisted metal strip. Stretching or elongation of this strip causes rotation of 
the central portion, in exactly the same way as the stretching of the twisted 
cord sets up rotation of the cardboard disc of the mechanical toy illustrated 
in fig. 2 . 1 5 A. A delicate pointer is attached to the central portion of the twisted 
metal strip ; thus very small linear movements of the plunger are recorded 
by this pointer on a suitably calibrated scale. 



MEASUREMENT 



41 



A wide variety of high magnifications are possible using the simple prin- 
ciple outlined. We owe the introduction of this type of instrument to a Swed- 
ish engineer, H. Abramson, who was responsible for its design, and to the 
well-known precision engineering concern of Messrs. C. E.Johansson, who 
manufactured it. 

2.7.5 Sigma comparator 

This is a British-designed and British-manufactured comparator of con- 
siderable popularity. The type shown in fig. 2 . 16 is of relatively simple design 
with regard to the external features of the instrument, as comparators are 
available capable of carrying out several checks on the one component. 



Twisted metal strip 



Light glass pointer 




— Plunger 



Fig. 2.15. — Twisted-strip Comparator. 

The type illustrated is available with a choice of scale ranges. A typical 
example is a measuring range of plus and minus 0.07 mm, with scale gradua- 
tions of 0002 mm. As the width of one division on the scale is 2 mm, and 
equivalent to a movement of the plunger of 0002 mm, the magnification of 
the instrument is 1000 : 1 . 

An important feature of this instrument is that the pointer, which is dead 
beat, is actuated by downward movement of the plunger, thus eliminating 
the possibility of damage to the mechanism arising from excessive upward 
blows on the plunger. Both the contact tip and worktable are interchange- 
able, according to the shape of the work to be checked, and these comparators 
are available with vertical capacities from 150 to 600 mm ; that is to say com- 
ponents up to 600 mm in height can be checked. Note the provision of limit 



42 



WORKSHOP TECHNOLOGY I 



indexes, or tolerance pointers as they are more commonly called, allowing 
the use of relatively unskilled operators to work to close limits when checking 
the accuracy of machined dimensions. 

There are of course, many other types of mechanical comparators in use, 
but the types chosen are a good indication of both the ingenuity of design and 
the manufacturing precision required for the production of an efficient and 

reliable comparator. 

Six scale ranges 
magnifications x 300 
to x5000 

Tolerance indicator 
adjustment 




Fine vertical 
adjustment 



Interchangeable 
contact tips 



Alternative types 
of tables 



Fig. 2.16.— Sigma Mechanical Comparator. 

2.8 Electrical comparators 

Some very great advantages are offered by the use of electrical com- 
parators. Mechanical devices, as we have seen, may be actuated by levers, 
gears, racks and pinions. All of these are subject to the effects of wear and 
friction, which are likely to affect the accuracy and useful life of the instru- 
ment. Electrical comparators, on the other hand, by their very nature will 
possess a minimum of moving parts ; thus we can expect a high degree of 
reliability from these instruments. 



MEASUREMENT 43 

In general, two important applications of electrical comparators are of 
the greatest interest : 

(i) the use of electrical comparators as measuring heads, 
(ii) the use of electrical gauging heads to provide visual indication as to 

whether a dimension is within the limits laid down. 
The first application is of great value when very precise measurements are 
required; say the checking or comparison of workshop slip gauges against 
inspection slip gauges. The second application is used not as a method of 
determining a linear dimension to within plus and minus 0-02 micrometres, 
but to indicate with a green light if a dimension is within the limits. An under- 
size dimension is indicated with a red lamp ; an oversize dimension with a 
yellow one. Once again it is no longer necessary for the operator to be aware 
of the actual tolerances on the dimension. Provided the instrument is cor- 
rectly set, the placing of the component under the plunger of the gauging 
head is all that needs to be done. The signal lamps provide instant and positive 
indication of the acceptability of the dimension under test. 

2.8.1 Principle of the electrolimit gauge 

Fig. 2.17 illustrates in a simple manner the principle of the electrolimit 
gauge or measuring head. Vertical movements of the plunger are transmitted 
to an armature, which is suspended, as shown in the diagram, on thin metal 
strips. At the left-hand side of the armature it will be seen that it lies between 
two electromagnetic coils A and B. These coils form two arms of an a.c. 
bridge circuit. 

Any movement of the armature between the two electromagnetic coils sets 
up out-of-balance effects, which are recorded by a microammeter. Provided 



Electro magnets 

-Armature 



[— Thin steel strips 



o A I o 

°4_11U 




Plunger 



Fig. 2.17. — Principle of the Electrolimit Measuring Head. 



44 



WORKSHOP TECHNOLOGY I 



the microammeter is calibrated in terms of the displacement of the plunger, 
direct reading of extremely small movements of the plunger is readily 
achieved. A front view of the complete instrument is shown in fig. 2.18. Note 
that the recording head is a separate unit, and that a supply of mains voltage 
is required. Fluctuations of up to 15% have no effect on the accuracy. 

A great advantage possessed by this electrical comparator is the dual 
magnification available. A simple switching arrangement enables a second 
magnification to be obtained, exactly double the first. Thus, assuming the 
instrument is being used with a magnification of 5000, it is a simple matter 
to increase this to 10 000. Even with the first magnification, the measuring 
range will be quite small, no more than 0-02 mm, whilst in the second case the 
range will be only 001 mm. 

Such is the accuracy or sensitivity of these instruments that they may be 
used with little trouble to check the accuracies of slip gauges and other 
measuring standards. (\ 

Comparator height cr ' 

adjustment handle J 



Recording 
head 



Reading 



Electrical 
mains— = 




j j/ Component 
under test 

Work table 



Fig. 2.18. — Electrical Comparator. 



2.8.2 Visual gauging heads 

The purpose of these heads is to give a visual indication, using small 
coloured signal lamps, of the acceptability of an engineering component 
with regard to the dimension under test. Clearly an electrical principle is 
involved, which may be simply described as follows, with reference to fig. 
2.19. Vertical displacement of an interchangeable plunger causes movement 
of the rod C either to the left or right, as shown in the diagram. A and B are 
electrical contacts, capable of precise adjustment in the direction of the 
arrows; a micrometer device is available. 



MEASUREMENT 



45 



In the position shown, that is to say with the rod in mid-position between 
the contacts A and B, the dimension under test is within the limits. If the 
dimension is oversize, the rod C moves to the right and makes contact with B. 
Immediately the top red lamp is illuminated. Likewise if the dimension is 
undersize the rod moves to the left, making contact with A and illuminating 
the yellow lamp. 

Note that the actual magnifying device is not shown on the diagram; 
levers and thin steel strips, together with knife-edge seatings, are employed. 

Micrometer 
adjustment 




Component 



77777777777; 



Fig. 2.19. — Principle of a Visual Gauging Head. 

With various detachable plungers, there is practically no limit to the 
application of this instrument. Fig. 2.19 illustrates the visual gauging of a 
single dimension, but we may apply the principle shown to several dimensions 
simultaneously. This technique is shown diagrammatically in fig. 2.20. 

2.8.3 Multi-gauging machines 

The component in fig. 2 .20 is shown having four diameters visually checked 
simultaneously. The component is set in a hand-operated carrier slide and 
pushed into the gauging station. A glance at the indicating panel will reveal 
whether the four diameters under test are within the limits laid down. If so 
the four centre green lights will signal, and the operator will remove the 
acceptable workpiece and replace it with another. 

Perhaps it is now evident to the student that we have come a long way in 
this matter of precision measurement. We see now the complete removal of 
the human element, for it is not difficult to arrange for automatic loading of 
components into the gauging station. It is not difficult, too, to arrange for 

\ 



46 WORKSHOP TECHNOLOGY I 

automatic rejection of undersize components, and retention, for subsequent 
salvage, of oversize components. Not only diameters, but also internal 
dimensions and heights, can be checked in the manner just described. 
Machines are available capable of the visual checking of over 30 dimensions 
on a fairly large-size workpiece. 

The setting, maintenance and care of machines such as these are most 
certainly the province of both mechanical and electrical technicians, and it is 
certain that great strides are being taken in the development and adaptation 
of these machines. 

Component 



Gauging unit 



L. 



KssKassml 




- OK ♦ 

OOO 



- OK ♦ 
OOO 



- OK + 

OOO 



- OK ♦ 

OOO 



Indicating panel 



Fig. 2.20. — Application of the Multi-gauging Device. 
2.9 Optical comparators 

There are many types of optical comparators in use, but all of them operate 
on one of two main principles : 
(i) the use of the optical lever, 
(ii) the use of an enlarged image. 

2.9.1 Optical magnification of movement 

An optical comparator is a device that provides considerable magnifica- 
tion of small linear movements of a measuring plunger. The principle adopted 
is that of the optical lever, and although the design of optical comparators 
varies considerably, the principle involved remains essentially the same. 

The principle of the optical lever is simply illustrated in fig. 2 .2 1 . A beam of 
light AC is directed onto a mirror, as shown at A, and is reflected onto the 
screen, appearing at O as an illuminated dot. Note that the angle at which 
the beam strikes the mirror is equal to the angle at which the beam is re- 
flected from the mirror ; both angles are measured from the normal, that is 
from a line projected at 90 to the surface of the mirror. 



MEASUREMENT 47 

At B we see the effect of vertical movement of the plunger, which causes 
the mirror to tilt on the pivot shown. Note that the reflected ray of light has 
now moved through the angle shown as 2a; this is twice the angle of tilt 
introduced by the plunger movement. The illuminated dot now moves to B ; 
thus a linear movement h of the plunger produces a movement of the dot 
equivalent to the distance OB on the screen. 



Normal 



Lamp%) 



V 

/ 
/ 



^^^MJ 



Pivot 



®l 




Mirror 



Plunger 



Fig. 2.21. — Principle of the Optical Lever. 

The magnification of the device shown may be calculated as follows : 
Because the angle of mirror tilt will be small, we may consider the angle in 
terms of radian measure. Let d equal the distance of the fixed pivot from the 
centre line of the movable plunger, and h equal the vertical displacement of 
this plunger. 

Then from radian measure, 



a radians = -, 



Similarly 

Thus 

and 



2a radians = 



OB 
CO 



,. 2h 

2a radians = -• T 

a 



OB 
CO 
OB 

h 



2h 
d 
2 CO 



Now the magnification of a comparator, as we have seen, is the ratio be- 
tween the distance moved by the indicating pointer and the displacement 
of the measuring plunger. 



48 WORKSHOP TECHNOLOGY I 

Because OB is the distance moved by the spot of light, we may consider, 
and indeed use, this spot of light as an indicating pointer. With h as the 
distance moved by the plunger, the magnification of the device must be 
OB/A, and this ratio is equal to 2 CO jd. 

7 It is now clear that as CO represents the distance of the screen from the 
tilting mirror, the greater this distance, the greater will be the magnification 
of the instrument. Also the smaller the distance between the fixed pivot and 
the centre line of the plunger, the greater will be the magnification. 

Many systems are in use to satisfy the requirements outlined above, and 
the following example is but one of a wide choice of optical-comparator 
operating principles. 

2.9.2 Application of the optical lever plus a mechanical lever 

Fig. 2.22 shows a typical application of the optical lever principle combined 
with a simple mechanical lever. The instrument which makes use of the 
principle shown is quite suitable for shop use ; it is of rigid construction, simple 
to operate, reliable in use and ideally suited for the checking of linear dimen- 
sions under mass-production conditions. 



Dimensions in nun 




Plunger 

I Work I 
U. table ^r^-i 



Tolerance pointers 

Fig. 2.22.— Principle of an Optical Comparator. 



Referring to fig. 2.22, we can see that downward movement of the plunger 
tilts the mechanical lever about its pivot P. Note that a mechanical magnifica- 
tion of 1 o : 1 is achieved with the dimensions given. The end of the lever shown 
as H causes a small mirror to tilt, thus deflecting a ray of light emanating from 
an electric bulb. From here on, the optical lever principle applies, and a 
green filter provides a clear visual pointer on the translucent screen. 



MEASUREMENT 

A view of this screen is shown at fig. 
2.22B. The overall length is about 150 
mm, and the magnification of the in- 
strument is 1 000 : 1 , so that the measur- 
ing range is plus and minus 0-075 mm. 
Note the use of tolerance pointers. 

A pictorial view of this comparator is 
given in fig. 2.23. 

2.9.3 Use of an enlarged image 

Optical comparators which make use 
of the enlarged image principle are com- 
monly known as optical projectors. 

The technique underlying the use of this 
measuring device is in accordance with 
our often stated rule of measurement ; 
namely the determination of an un- 
known value by comparison with a 
known standard. 

We may define the purpose of an 
optical projector as follows : to compare the shape or profile of a relatively 
small engineering component with an accurate standard or drawing much 
enlarged. The optical projector throws onto a screen an enlarged image of 
the component under test; the principle is illustrated in fig. 2.24. 

Note that the rays of light from the lamp A are collected by the condenser 




Fig. 2.23. — Optical Comparator. 




Condenser ^ , ^bl, v 
Source of ,,\ < ±m/ 



® 



Component 



Fig. 2.24. — Principle of the Optical Projector. 



50 WORKSHOP TECHNOLOGY I 

lens B, from which they are transmitted as a straight beam. It will be seen in 
the diagram that the threaded component E has been placed between the 
condenser lens and a projection lens C. 

In this way the beam of light is interrupted, and a magnified image appears 
on the screen as shown. A sharp or well-defined image is obtained by focusing, 
or adjustment of the distance between the component and the projection 
lens. Once again the magnification of the system will be equal to the size of 
the projected image divided by the size of the component ; such magnifica- 
tions are arranged in relation to the focal length of the objective lens and its 
distance from the screen. 

The magnification may vary from i o to i oo. If we assume that a magnifica- 
tion of i oo is used in the arrangement shown in fig. 2 .24, a drawing is made of 
the thread profile, with all dimensions one hundred times full size ; thus the 
profile of the thread is magnified 100 times, allowing a comparison of the 
resultant image with the accurately produced master drawing. 

It is not difficult, using a micrometer device or slip gauges, to control the 
linear movement of the worktable on which the holding centres are located. 
This means that variation of the magnified image from the master drawing 
can be determined accurately by noting the movement required along the 
axis shown by the arrow X in the diagram. 

It is essential that the worktable be rotated through an angle equal to the 
helix angle of the thread, as shown in fig. 2.24B. Because the distance from 
the projection lens to the screen has a direct effect on the amount of mag- 
nification obtained (that is to say, the greater this distance the greater the 
magnification), mirrors are often used to increase this distance without mak- 
ing the projector unnecessarily bulky and causing it to occupy undue floor 
space. 

2.9.4 The toolmaker's microscope 

A toolmaker's microscope is essentially a microscope that has provision for 
the fitting of various standards, against which a magnified image of the pro- 
file under test can be compared. These standards are in the form of graticules 
— glass discs on which are engraved thread angles or other reference lines. 

A typical graticule is simply shown in fig. 2.25A; this is one used to check 
rack-tooth angle form. A special device allows rapid setting of the line shown 
as P parallel to the direction of the worktable travel, when the circular scale 
is set at o°. 

The initial setting, shown at A, illustrates an image of the rack tooth with 
the axis of the rack parallel to the worktable movement. At this position the 
circular scale reads zero. At B we see the technique adopted to determine the 
rack-tooth angle. The view shown represents the result of rotation of the work 
in order to bring the surface of the tooth, shown as RT, parallel with the 
graticule line S. A separate microscope may be used to determine to a high 
degree of accuracy the angle of rotation through which the protractor unit 
carrying the graticule has rotated. 

In this way very accurate determination of the accuracy of small rack teeth 
may be made. Typical examples of the need for this sort of measurement are 



MEASUREMENT 



5 1 



Zero reading 




Circular scale 

Fig. 2.25. — Graticules for Toolmaker's Microscope. 

provided by the racks, pinions and levers used in the mechanical comparators 
previously described. 

There are, of course, many other measuring operations that can be carried 
out with the aid of a toolmaker's microscope. Graticules having a wide range 
of thread profiles are available, allowing the checking of small screws, screw 
gauges and thread-cutting tools. Fig. 2.26 illustrates in a simple manner a 
typical toolmaker's microscope. Note that the worktable has micrometer 
adjustment. 




— 0=L 



2l 



Fig. 2.26. — Toolmaker's Microscope. 



52 WORKSHOP TECHNOLOGY I 

Summary 

Precision measurements are an essential feature of all toolrooms and 
inspection departments. It is a general principle of engineering manufacture 
that when precision components are required, machine tools, jigs, fixtures 
and press tools are used in order to produce them. In the same way, the accur- 
acy of a drop forging or pressure die casting depends on the dies used ; pro- 
vided the dies are manufactured and checked to within the limits of tolerance 
laid down 'on the drawing or blueprint, the dimensional accuracy of the 
forging or casting remains constant, subject only to wear or deterioration 
of the dies. We have seen that precision dimensions require the use of accur- 
ate working standards such as slip gauges, precision rollers and balls. 

Provided the student has a good working knowledge of geometry and 
trigonometry, a wide range of measuring techniques is possible, with linear 
accuracies of within plus and minus two micrometres. These techniques, 
as we have seen, involve the principle of direct measurement, and the student 
will no doubt discover in the course of his laboratory work that a great deal 
of skill, patience and time is required if accurate results are to be obtained. 
All these factors are dependent on what we may consider as the human 
element. 

It is not enough that a dimension is carefully checked on a single occasion ; 
close tolerances demand precision checks at fixed intervals, especially if the 
component is mass-produced using an expensive machine tool such as a 
centreless grinding machine. 

Precision checks of this nature must be carried out quickly, yet with the 
achievement of consistent and precise results, preferably with the use of un- 
skilled or semi-skilled personnel. This work is possible with the use of com- 
parators, or the principle of measurement by comparison. The dimension 
required is determined by comparison against a known standard ; any devia- 
tions are suitably magnified and shown on a dial or scale. The principles of 
magnification may be mechanical, electrical, optical, or even pneumatic. A 
wide range of instruments embodying one or more of the above principles is 
available, and it is important that the mechanical engineering technician 
has some knowledge of the principles involved. This is the true purpose of 
technology, namely the application of scientific principles to manufactur- 
ing techniques or processes. 

EXERCISE 2 

i With a practical example, explain the need for the precision measurement of an engin- 
eering component. 

2 What is the essential difference between direct measurement and measurement by 
comparison ? 

3 Why are end measuring bars necessary for the direct measurement of certain linear 
dimensions? What precautions must be taken when using these bars? 

4 With the aid of a neat sketch, show a typical measuring set-up requiring the use of 
working standards such as slip gauges, precision rollers or balls. What degree of accuracy 
can be expected from the set-up shown ? 

5 What are the limitations when determining linear dimensions using direct measure- 
ment techniques involving working standards? A centreless grinding machine produces inlet 



MEASUREMENT 53 

valves for a motor-car engine at the production rate of 720 per hour, with the diameters 
ground to within plus and minus two micrometres (see Workshop Processes 2, fig. 90). Describe 
a suitable measuring technique for the checking of the diameter at three different points at 
intervals of 10 minutes. 

6 Explain the purpose of a comparator as used in engineering measurement. What are 
the advantages offered by the use of comparators when making precision linear checks? 

7 With a neat diagram illustrate the principle of a typical mechanical comparator ; show 
clearly the method adopted to obtain magnification of the plunger movement. 

8 Describe a typical comparator used on a production line, providing visual indication 
by means of coloured signal lamps whether six separate diameters are within the limits. 

9 Make a neat sketch of a component that would require an optical projector in order to 
determine whether it possesses the required accuracy. 

10 What is meant by the principle of the optical lever? With a neat diagram show the 
application of this principle to a typical optical comparator of the vertical type. 



O Inspection 

3.1 Introduction 

The difference between measurement and inspection was fully outlined in 
Workshop Processes 2, but perhaps some revision will assist at this stage. 

We can define measurement as a specific attempt to determine a linear or 
angular dimension, or the non-linear functions of flatness, roundness, con- 
centricity or alignment. While it is not possible to obtain exact results, the 
use of the equipment outlined in the previous chapter allows a high degree of 
accuracy. In all measuring techniques a quantitative result is achieved ; in 
other words we get an answer to the measuring problem. For example, if we 
measure the diameter of a piston for a motor-car engine, we must arrive at a 
certain linear dimension, say 75-05 mm. 

The accuracy of this dimension with respect to the precise or exact dia- 
meter of the piston depends on the type and precision of the measuring equip- 
ment used, together with the skill and experience of the person making the 
measurement. Inspection, on the other hand, is not concerned with the 
determination of dimensions, but with the control of these dimensions. 

3. 1. 1 Dimensional control 

This is the true purpose of engineering inspection. It is an essential process 
if the mass-production of engineering components is to produce components 
which not only are of reasonable quality, but also have the important property 
of a long and reliable life. Before the advent of modern manufacturing tech- 
niques, engineering assemblies such as steam engines, pumps and other 
mechanical devices were hand fitted. The fitting of each individual part to 
its mating part was a long and expensive business requiring highly skilled 
and highly paid craftsmen. In the event ofbreakage or damage at a later date, 
the repair proved both costly and time-consuming. The new component 
would require machining and fitting, perhaps demanding the services of the 
same craftsman who fitted the original assembly. Such an assembly was an 
example of the manufacturing process called custom building; each 
assembly, although capable of performing exactly the same task as another 
assembly, was a separate unit. No part or component could be interchanged. 

3.1.2 Mass-production 

Mass-production can only be achieved through rigid application of the 
principle of dimensional control. Although the first seeds of the mass-produc- 
tion technique germinated in Europe with the manufacture of completely 
interchangeable musket locks, it was the American inventor Eli Whitney who 

54 



INSPECTION 55 

first introduced the principle of mass-production to the manufacture of 
engineering components. As early as 1819, muskets and pistols were mass- 
produced in American arsenals. The importance of having components for 
articles such as these completely interchangeable needs no emphasising. 
The mass-production technique was quickly adopted for the manufacture 
of clocks and other domestic appliances. 

Meanwhile mass-production was also being appreciated in Great Britain. 
By 1857 the Royal Small Arms Factory at Enfield was producing 1000 rifles 
per week; this figure was later increased to 2000. Over 700 operations were 
required in the manufacture of a single rifle, yet each part was completely 
interchangeable. All this was achieved by ensuring that each dimension 
was within the limits laid down ; in other words interchangeability is only 
possible with the application of dimensional control. 

3.2 The need for limit systems 

Consider the assembly illustrated in fig. 3.1. The needle valve shown at A 
is a close slide fit in the phosphor-bronze bush shown at B. This bush is a 
drive fit in the body of the instrument, as is the hardened and tempered steel 
bush shown at C. Movement of the valve in the direction of the arrow X seals 
the orifice at O, thus regulating the pressure of air or oil through the orifice. 

Clearly if this assembly is to be mass-produced cheaply and efficiently, and 
the customer or user is to be provided with a spare-part service, we must 
decide on the sort of fits present in the assembly. Of greater importance, we 
must decide on the dimensions that will produce the sort of fits needed. If 

-P hosp hor bronze bush 

Needle valve 




A 


Needle 




valve 


B 


Bush 


C 


Bush 


D 


Body 



(Reduced) 



Fig. 3.1.— Needle Valve Assembly. 



56 WORKSHOP TECHNOLOGY I 

every engineering designer were allowed free rein in this matter of deciding 
the sizes of mating components to produce different kinds of fits, then com- 
plete and utter chaos would result. 

It is the purpose of a limit system to establish the types of fits most likely 
to be needed in engineering manufacture, and to recommend the dimen- 
sions of the mating parts. 

3.2.1 BS 4500:1969, "ISO limits and fits" 

This British Standard Limit System supersedes BS 1916:1953, "Limits 
and fits for engineering". Following a complete review of the whole field of 
limit systems, it was decided to develop a new limit system based on the ISA 
system. (Formulated by the International Federation of National Standard- 
izing Associations, more commonly known then as ISA and now as ISO, 
the ISA system had long been used with much success on the Continent.) 
Briefly the system makes use of the following important items : 
(i) fundamental tolerance, 

(ii) fundamental deviation. 

3.3 Tolerance grades 

Let us consider once more the assembly shown in fig. 3.1. If this assembly 
is to be mass-produced, then it is an accepted rule that the tolerances on the 
component parts be as large as possible. This is because small tolerances 



2500! 
1 2 SO 



"folerance 
OOOImmQum) 



Tolerance 
qualit y 




A 12 25 38 50 64 76 89 114 Q 
Diameter in mm 

Fig. 3.2. — Degree of Accuracy expected from Machine Tools. 



INSPECTION 



57 



place a heavy burden on the manufacturing cost of the components ; ex- 
pensive machine tools are needed, together with delicate and costly measur- 
ing equipment. Unless the greatest care is taken scrap rates will be high, and 
this must add to the cost of the finished article. Ideally, dimensions should 
be machined to an exact size, but this is not possible, and the design engineer 
must decide on the largest tolerance permissible, keeping in mind the quality 
or useful life of the completed assembly. 

3.3.1 Choice of tolerance grades 

BS 4500 provides eighteen grades of tolerance, designated IT01, ITo, 
ITi to IT16. The choice of a particular tolerance is governed by the earlier 
stated maxim of economic manufacture allied with satisfactory performance. 
Economic manufacture can be achieved only with the use of machine tools, 
and the degree of accuracy which can reasonably be expected from these 
machine tools determines the value of the tolerance grade. 

This degree of accuracy is shown in graphical form in fig. 3.2. On the 
vertical line AB we see the tolerance in units of one-thousandth of a milli- 
metre; on the opposite vertical line CD we see the ISO tolerance grades or 
quality. Clearly the higher the degree of accuracy of which the machine 



IT 


Class of work 


Machine tools 


16 

*5 
14 

*3 
12 
11 

10 

9 
8 

7 
6 

5 

4 

3 
2 
1 

01 


Sand casting, flame cutting 
Stamping (approximately) 
Die casting or moulding ; rubber moulding 
Press work, tube rolling 
Light press work; tube drawing 
Drilling, rough turning and boring, pre- 
cision tube drawing 

Milling, slotting, planing, metal rolling 
and extrusion 

Worn capstan or automatic lathe, hori- 
zontal and vertical boring machines 
Centre lathe turning and boring, reaming, 
capstan or automatic in good condition 
High quality turning, broaching, honing 
Grinding, fine honing 
Machine lapping, fine or diamond boring, 
fine grinding 
Gauges, precision lapping 

Good quality gauges 
High quality gauges 
Workshop standards and gauges 
Inspection standards and gauges 
Work of the highest quality 


Flame cutting machine 

Drop forging hammer 

Die casting machines 

Machine presses 

Machine presses 

Drilling machines, 

lathes 

Milling, slotting and 

planing machines 

Capstan and automatic 

lathes, borers 

Lathes, capstan and 

automatic 

Lathes, honing and 

broaching machines 

Lapping, boring and 

grinding machines 

Precision lapping 

machines 



Table 3.1 Tolerance grades (ISO series of tolerances) 



58 WORKSHOP TECHNOLOGY I 

tool is capable, the smaller is the tolerance. Note also that the horizontal line 
AC shows the effect on the tolerance of increasing diameter. Table 3.1 gives 
the fundamental tolerances, together with the machine tools used and the 
type of work. 

Much depends, of course, on the condition of the machine tools used, and 
it is no exaggeration to state that the greater part of reject work is due to the 
inability of the machine tool or the tooling arrangement to hold the required 
tolerance. This is particularly so in the drilling, boring or reaming of holes, 
or for that matter when producing a hole by any other method . For this reason 
BS 4500 recommends that in the case of a shaft mated to a hole, the hole is 
allocated a tolerance one grade coarser than the shaft. The production of 
shafts to close dimensional limits is a much better proposition than the pro- 
duction of holes of similar quality. 

It is important that Table 3. 1 be used in conjunction with fig. 3.2. This will 
allow us to see the true value of a machine tool, that is to say its ability to 
produce accurate work. 

Let us take as an example the quality of tolerance allocated to a worn 
capstan lathe. This is IT9, and if we assume that work of 25 mm diameter is 
turned, then reference to fig. 3.2 shows that the tolerance is approximately 
0-05 mm. This point is ringed in the diagram. Thus it is unwise to attempt 
to work to (say) a tolerance of plus and minus 002 mm on a capstan lathe 
which is not in good condition. Much reject work will result, with much time 
spent by the capstan setter in his attempts to produce work within the dimen- 
sional tolerance of 0-04 mm, while the basic fault lies in the continued use 
of a machine tool which does not possess the accuracy needed for the class 
of tolerance given to the component. 

3.3.2 Derivation of the standard tolerance 

We have seen that the grade or quality of tolerance allocated to a dimension 
depends mainly on the class of work required, together with the type and 
quality of machine tool used. All standard tolerances are multiples of a 
basic tolerance unit called i. 

The value for i is calculated as follows : 

e(micrometres) = 045 x l/B+o-ooi x D. (D in millimetres) 
In each of the above cases D is the geometric mean of the diameter steps 
involved. Tolerance steps progress geometrically to allow for the increased 
expansion and deformation which affect both gauges and workpieces as 
dimensions become greater. The effect of the geometric progression is to 
make each successive tolerance grade about 60% greater than its predecessor. 

Fig. 3.3 shows a diagrammatic representation of the clearance fit illus- 
trated in fig. 3.1. This is a simple way of showing the definitions of the terms 
used. Note that the tolerance is shown as a dark band on the top of the valve 
guide, while in actual fact it is, of course, applicable to the whole diameter. 
Note also that we have given this shaft a tolerance grade of 6, in accordance 
with the use of Table 3.1. Before we turn to the method of determining the 
amount of this tolerance without reference to fig. 3.2, let us examine the 
second important item of BS 4500, namely the fundamental deviation. 



INSPECTION 




Centretess ground Tolerance grade - 6 

Fig. 3.3. — Clearance Fit. 



Deviation H=0 



vlvlr 



IT 9 



1 



m 



IT IO 



I, ,■ 

//A Zero line 



I 



T 

IT 7 



mm 



IT 6 
-Deviation f 

Fig. 3.4. — Fundamental Deviations. 



3.4 Fundamental deviation 

The fundamental deviation determines the type of fit obtained when 
mating a shaft to a hole. The quality of the fit is determined by the tolerance 
grade, hence the actual nature of the fit results from the magnitudes of the 
fundamental deviations and tolerances on the mating parts. There are 27 
different deviations for both holes and shafts, and fig. 3.4 illustrates the 
association of fundamental deviations and tolerances. Note that the shaded 
areas represent the tolerances of H holes, all of which have zero deviation, 
that is to say the low limit of the hole lies on the zero line. On the other hand, 
the f shafts shown have a minus deviation with varying tolerances according 
to the quality of fit required. Thus the smaller the deviation on the shaft the 
closer is the fit, and the smaller the tolerance grade the better is the quality. 

3.4.1 Designation of fundamental deviations 

Fundamental deviations are designated by the letters of the alphabet, 
capital for holes and lower case for shafts ; thus the letter "a" indicates a large 
negative deviation, whilst letter "z" represents a large positive deviation. 
All the letters of the alphabet are used, with the exception of i, o, 1, q and w. 



6o 



WORKSHOP TECHNOLOGY I 



The fundamental deviations are for holes : 
A B C CD D E EF F FG G H JS J K M N P R S T U V X Y Z ZA ZB ZC 
for shafts : 
a b c cd d e eff fg ghjsjkmnprst uvxyzzazbzc 

The complete designation of the limits of tolerance for a shaft or hole re- 
quires the use of the correct letter indicating the fundamental deviation, 
followed by a suffix number indicating the appropriate tolerance grade. 
Thus, as all holes in engineering have a recommended zero deviation, a hole 
with a tolerance grade IT 7 is designated H7, and similarly a shaft of "f 
deviation and tolerance grade IT 8 is designated f8. 

To indicate a fit, the hole diameter must first be stated, followed by the 
hole limits, then the shaft limits. For example, a 50 millimetre diameter hole 
designated H6, mated to a shaft p5 is indicated thus : 

50 H6-P5 or 50 H6/P5 

Such indications should appear on design drawings only ; on all working 
drawings the actual limits of both hole and shaft must be explicitly stated. 

3.5 Selection of fits 

Although BS 4500 provides a comprehensive system covering diameters 
from 3 mm to 3 1 50 mm diameter, for ordinary engineering purposes only a 
relatively small number of fits are required. It is recommended that H holes 
only be used. All H holes have zero deviation, as shown in the diagram of 
fig. 3.5; note the curve (shown in dotted line) resulting from the geometric 



JJn'rts - O025mm - 2S)jun Curve 



Zero line 




HM DRILLED 

H 9 BORED 

H 8 REAMED 

H 7 BROACHED 



Tolerance zones for H holes (30 - SOmm diameter) 

Fig. 3.5. — Recommended Holes for Ordinary Engineering Purposes. 



INSPECTION 



6l 



arrangement of the tolerance zones. Note also that these holes have unilateral 

tolerances, that is to say plus plus. 

Reference to fig. 3.5 shows that for ordinary engineering purposes only 

four grades of holes are required, as listed below : 
Grade Produced by 

H7 high quality boring, broaching, honing 
H8 centre lathe boring, reaming, good quality capstan work 
H9 horizontal and vertical boring, worn capstan work 
H 1 1 standard drilling 



ES = Upper deviation Unit = 0001 mm 
EI = Lower deviation = i/im 


Nominal sizes 


Hi 


m 


H 9 


Hi 1 


Over 


Up to 

and 

including 


ES 

+ 


EI 


ES 

+ 


EI 


ES 

+ 


EI 


ES 

+ 


EI 


mm 


mm 


















— 


3 


10 





14 





25 





60 





3 


6 


12 





18 





30 





75 





6 


10 


*5 





22 





36 





90 





10 


18 


18 





27 





43 





no 





18 


30 


21 





33 





52 





130 





30 


50 


25 





39 





62 





160 





50 


80 


30 





46 





74 





190 





80 


120 


35 





54 





87 





220 





120 


180 


40 





63 





100 





250 





180 


250 


46 





72 





"5 





290 





250 


315 


52 





81 





130 





320 





3*5 


400 


57 





89 





140 





360 





400 


500 


63 





97 





155 





400 






Table 3.2 

Limits of tolerance for selected holes (BS 4500:1969) 



62 



WORKSHOP TECHNOLOGY I 



It must be appreciated that the tolerance zones shown in fig. 3.5 are applic- 
able only to holes within a diameter range of from 30 mm to 50 mm, as in- 
dicated on the diagram. It is suggested by BS 4500 that diameters within a 
range of from 3 mm to 500 mm will meet the needs of an average engineering 
works. Table 3.2 shows the British Standard limits for H holes within this 
range. 

Note that the units given in the table are equal to o-ooi mm. and that the 
tolerance increases with the diameter. Note also that ES means upper 
deviation, and EI means lower deviation. 



All dimensions in mm 




Tolerance =0027mm 



HUhigh 

L r low 



Fig. 3.6. — Bronze Bush for Needle Valve Assembly 



We may take as an example the hole in the bronze bush previously shown 
in fig. 3.1 and also as a pictorial view in fig. 3.6A. The basic diameter of the 
hole is 15 mm. If as planning engineers we decide to produce this bush on a 
good quality capstan lathe, then reference to Table 3.1 shows that a tolerance 
grade of 8 is recommended. As already stated, the tolerance is also dependent 
on the diameter size, and reference to Table 3.2 gives the upper deviation 
for a$i5 H8 hole as 27 units. 

This means that the limits of size within which the hole must be machined 



l 5 



+ o-027mm 
+ 



Fig. 3.6B shows an orthographic drawing of the bush, giving the limits of 
size for the machined hole, while at fig. 3.6C is shown a conventional repre- 
sentation of the hole. Note that the tolerance zone for a hole is cross-hatched, 
while that for a shaft is shown in black. 



INSPECTION 63 

All that is now required is the selection of a shaft size that will produce the 
required fit between the needle valve (shaft) shown also in fig. 3.1 and the 
H7 hole already chosen. 

3.6 Types of fits 
3.6.1 Clearance fits 

A clearance fit is obtained when the low limit of a hole exceeds the high 
limit of the shaft that is to mate with the hole. This means that the shaft must 
possess a negative deviation, as shown in fig. 3.3. 

Clearly the greater this deviation the coarser or slacker will the resultant 
fit be ; Table 3,3 shows the clearance fits suitable for the average engineering 
works as recommended by BS 4500. 




All holes hove zero deviation 



v/ 7 Close run 




4- 



Loose run 
Extra slack 

Clearance 



Shafts have minus deviation 
except shaft h which has 
zero deviation 



■0 



Transition 

— 1> 



Table 3.3 Selected clearance fits 



Example of a clearance Jit 

We see from the above table that a fairly wide choice of clearance fits is 
available, ranging from an extra loose running fit to a slide fit. It is the duty of 
the engineering designer to decide on the actual type of clearance fit, and 
designate it by using the recommended symbols. Fig. 3. 7 A illustrates a 
typical example of a loose running fit, shaft "d". 

This combination of Hg/dio is suitable for most loose running fits such as 
loose pulleys or gland seals. The diagram shown at fig. 3.7B illustrates the 
essential conditions for the type of fit required. Note that the tolerance grade 
for the hole is given as H9; reference back to Table 3.2 shows us that the 
tolerance is plus zero, plus 0-062 mm, the diameter of the hole lying in the 
30 mm to 50 mm range. 

With a tolerance grade of 9, or 0-062 mm tolerance, the machining of this 



64 WORKSHOP TECHNOLOGY I 

hole will present no problem. If the pulleys are to be mass-produced, it is 
certain that a capstan lathe will maintain this kind of accuracy under pro- 
duction conditions, and, as we have already pointed out, much scrap or 
wastage results if the machine tool used is unable to produce the work con- 
sistently to the tolerance. 

In this example the shaft is one grade coarser than the hole, but for better 
quality fits it is better to make the hole one grade coarser than the shaft. 



ALL DIMENSIONS IN MM 



045 Hg-djo 





Shaft 
tolerance 



Pulley rotates on shaft 



Fig. 3.7. — Practical Example of a Loose Running Fit. 

3.6.2 Transition fits 

Some confusion appears to exist with regard to the definition of a transition 
fit. Perhaps reference to fig. 3.8A will assist in clearing away some of the 
problems associated with a correct appreciation of this type of fit. 

At A we see the conditions necessary to produce a transition fit. If, now, the 
hole is on its high limit and a shaft on low limit is mated to it, then clearly, 
as shown in fig. 3.8B, a clearance fit must result. On the other hand, if the hole 
is on its low limit and is assembled to a shaft on high limit, a slight interference 
fit results, as shown in fig. 3.8C. 

Do not be misled by the exaggerated proportion of the diagrams. The 
tolerances associated with transition fits are very small indeed, and the inter- 
ference is so slight that hand pressure is sufficient to cause entry of the shaft. 
The important point to remember, however, is that the type of fit obtained 
depends on the actual dimensions of the hole and shaft; in actual practice 
both hole and shaft will be somewhere around the middle limit. This means 
that the conditions shown in fig. 3.8A exist, and this is a good example of a 
true transition fit. Any variation of either hole or shaft within the tolerance 
range tends to produce a slight clearance or interference according to the 



INSPECTION 65 

direction of the tolerance variation. Thus the actual fit obtained will be a 
transition fit, liable to change from one type to another. 

Example of a transition Jit 

Fig. 3.8D shows a good example of a transition fit. The device shown is 

much used on machine tools equipped with leadscrews, rotation of which 

rHole tolerance ,, , 
H 7 -k 6 

Shaft 
tolerance 




^.HLshigh limit 
(£)lL»Iow limit 



Indexing dial disc 



Handwheel 





Slight 
interference 



Handwheel and 
indexing dial 
keyed to shaft 



02Omm 



Fig. 3.8. — Practical Example of a Transition Fit. 

gives rise to linear movement of a slide or worktable. Both handwheel and 
indexing dial disc are keyed to the leadscrew shaft, and because this assembly 
is used on a machine tool it is certain to be subject to considerable vibration. 



Push or 
Easy key 



Drive 



Holes hove zero 
deviation 




Clearance 



Interference 
{> 



Shafts have 
positive deviation 



Table 3.4 Selected transition fits 



66 



WORKSHOP TECHNOLOGY I 



A slight interference fit is recommended, and this is achieved with a com- 
bination of H7/k6, as shown on the diagram. Note that the tolerance on the 
hole is one grade coarser than that on the shaft; the hole limits are +0-02 1 
mm, +o mm, the shaft limits are +0-002 mm, +0-015 mm- 

This is only one of the transition fits recommended by BS 4500 ; Table 3.4 
sets out fuller details of the recommended types of transition fit. Once again 
the choice of the classification is the problem of the design engineer, but 
once a decision has been made on the type of transition fit required, applica- 
tion of BS 4500 is to be recommended. 

3.6.3 Interference fits 

An interference fit is obtained when a low-limit shaft exceeds the diameter 
of a mating hole on high limit. This means, that provided a batch of holes is 
machined to within the limits, all shafts taken from a batch also machined to 
within the limits will be interference fits with the holes. The amount of inter- 
ference determines the degree of force required to assemble or mate the 
shaft to the hole. Some care is required when considering the type of inter- 
ference fit required for a particular assembly. The quality of the surface 
finish of the mating parts, the size of the diameters, the metals from which they 
are made, all affect the quality of the fit obtained. 

BS 4500 recommends two interference fits, and the appropriate informa- 
tion is shown in Table 3.5 



Medium 
Light p^s 
Press 




Transition Interference 



Holes have 
zero deviation 



Shafts have 
positive deviation 



Table 3.5 Selected interference fits 



Examples of an interference Jit 

Interference fits are used as a cheap and efficient method ofjoining together 
two components. We see from Table 3.5 that a light press fit may be used ; 
this is our first interference fit. Its use in engineering manufacture is confined 
to the assembly of ferrous components which require removal for purposes 
of renewal or replacement at a later date. 



INSPECTION 67 

If the components to be joined are not likely to require separating at a later 
date, a press fit may be used. Typical examples are bearing bushes in alloy 
housings or castings. When severe gripping forces are required the parts may 
be shrunk to one another. This involves the heating, or alternatively the 
refrigeration, of one component to a suitable temperature, whereupon it is 
assembled to the mating part. When room temperature is restored powerful 
forces are brought into play, resulting in a permanent joint between the two 
components. Fig. 3.9 illustrates three typical examples of interference fits. 



Drill bush in jig plate 




02OH 7 -p 6 



® 
(Light press) 




Pump impeller 
shaft. Medium press 




V///////M 




Cylinder liner 
in block 



95H 7 -u & 



Fig. 3.9. — Examples of Interference Fits. 

Fig. 3.9,4 

A drill bush in a jig plate. The combination Hy/p6, although producing a 
relatively small amount of interference, is sufficient to give a press fit which 
can be dismantled and assembled when required without overstraining the 
parts. 

Fig- 3-9^ 

A pump impeller on a shaft. A medium press fit, using the combination 
H7/S6; can be driven home, and is sometimes referred to as a light drive fit. 

Fig. 3-9 c 

A cylinder liner in a cast-iron block. Classified as a heavy drive fit, pro- 
ducing a permanent or semi-permanent assembly between liner and block. 
Large sizes require heating and shrinking to avoid the possibility of damage 
which exists if we attempt to assemble cold. 



68 



WORKSHOP TECHNOLOGY I 



3.7 Use of BS 4500:1969 

All the necessary information, recommendations and relevant tables are 
to be found in the booklet BS 4500:1969, entitled "ISO limits and fits", 
issued by the British Standards Institution. Provided the mechanical 
engineering technician is familiar with the symbols used for both holes and 
shafts, it is a relatively simple matter to convert these to decimal dimensions 
and insert them on the working drawing. 

It is certain that the engineering designer will himself make the fullest use 
of BS Handbook No. 18, entitled "Metric standards for engineering". The 
fullest information is given with regard to recommended combinations of 
shaft and hole for various types of fits, together with suitable alternatives or 
second choices. While the actual dimensions of manufactured components 
are governed by considerations of quality, appearance and economy of 
manufacture, all of which the engineering designer takes into account before 
arriving at a theoretical size, much advantage is to be gained if preferred 
sizes are finally adopted. 

3.7.1 Preferred numbers 

Let us assume that a designer has calculated that the diameter of a pulley 
shaft should be 26-3 14 mm if it is to have the necessary strength to stand up to 
service conditions. Now 26-3 14 mm is an awkward size, and it is bad practice 
to put a number of this sort on an engineering drawing. ISO recommenda- 
tions for Metric Standards in Engineering are that the designer refers to a 
series of preferred numbers, and Table 3.6 shows an extract from the basic 
series of preferred numbers. 

It is clear that the R40 scale provides the nearest value to the 26-314 mm 
diameter of the pulley shaft, and the preferred number 265 can be chosen. 

The correct choice of scale must take into account the technical and 
economic factors involved in the manufacture of a component. The R5 



I 


2 


3 


4 


5 






Basic seri 


es 


Serial 
number 








R5 


Rio 


R20 


R40 


16 
18 


250 


250 


25-0 
280 


250 
265 
280 


l 9 








30-0 


20 




31-5 


3 J -5 


31-5 


21 








335 


22 






355 


355 


23 








37-5 


24 


40 


40 


400 


40-0 



Table 3.6 Extract from basic series of preferred numbers 



INSPECTION 69 

scale has wide steps and may lead to a waste of materials, while a closely spaced 
scale such as the R40 may involve increased costs in tooling or gauging. 

3.8 Limit gauges 

We have seen that dimensional control can be achieved with the use of 
comparators, and that these are especially helpful if the tolerance on the 
dimension to be checked is a small one. Another cheap and efficient method 
is provided by limit gauges, which can be used to test shaft and hole diameters, 
slot widths and depths, and many other dimensions occurring in the manu- 
facture of engineering components. It is the purpose of a limit gauge, as its 
name suggests, to ensure that no dimension is outside the acceptable limits. 

3.8.1 BS 1044: Part 1:1964, "Gauge blanks" 

We shall be concerned with gauges of three basic kinds : plug gauges for 
holes, and gap and ring gauges for shafts. BS 1044 covers all recommenda- 
tions with regard to the design of plug, ring and gap gauges. Tables give sizes 
for gauging members and handles according to the diameter range of the 
work to be tested. Recommendations are also made for the marking or 
stamping of the finished gauge. 

3.8.2 Application of limit gauges 

Let us consider the limit gauging required to ensure that the assembly 
shown in fig. 3.7A has been correctly machined, and discuss the different 
types of gauge with this purpose in mind. 

PULLEY 

SHAFT 




400 



All dimensions in mm 

Fig. 3.10. — Assembly requiring Gauges. 

Fig. 3.10 shows a drawing of both pulley and shaft. Referring to the BS 
tables for a 40 H8/f7 fit, we arrive at the following limits of size for the 
assembly : 



high limit hole = 40039 mm 
low limit hole = 40000 mm 



high limit shaft = 39-975 mm 
low limit shaft = 39*950 mm 



70 



WORKSHOP TECHNOLOGY I 



3.9 Plug gauges 

The gauge used for the checking of the hole is a solid-type cylindrical 
plug gauge. Three types of plug gauge are illustrated in fig. 3. 1 1 . 

At A. we see a single-ended plug gauge. If this type of gauge is used to 
check the H8 hole in the pulley two gauges are needed : a go plug gauge at 
40 mm diameter and a no go plug gauge at 40039 mm diameter. 

At B we see a double-ended plug gauge ; note that the no go member is 
shorter than the go member, thus allowing easy recognition between go and 
no go ends. 

At C we see a progressive plug gauge. This gauge requires the minimum 
of operations on the part of the gauge user, but its use is limited to through 
holes; it could not be used to check a blind hole. 



id 



2 mm 



-**+ 



© 



->— 



— K&ax&y 



m>w 



Fig. 3.1 1. — Solid-type Cylindrical Plug Gauges. 



3.9.1 Renewable-end plug gauges 

Although solid-type plug gauges may be hard-chromed and re-ground, 
thus avoiding scrapping the gauge on account of wear of the gauging dia- 
meters, considerable economies are possible by the use of renewable ends 
or handles. Two types of renewable-end plug gauges are in use : 
(i) taper-lock design, 

(ii) trilock design. 



INSPECTION 



7* 



Taper-lock design 

This type of renewable-end plug gauge is suitable for diameters up to and 
including 60 mm. Fig. 3.12 illustrates both the gauging member and the 
handle of a gauge used to check the H hole in the pulley. Note that a progres- 
sive gauging member may be used instead of the single go member shown in 
fig. 3. 1 2 A. Provided the tapers in both gauging member and handle are 
accurately machined and assembled, the taper lock is equivalent to a solid 
gauge with respect to rigidity or freedom from shake. 



Drift hole 




Taper I in 48 



Handle. 




Fig. 3.12. — Renewable-end Plug Gauges with Taper Lock. 

Trilock design 

This design was originated by the Pratt & Whitney Co., and the Taft- 
Peirce Manufacturing Co., both of America. Although it is protected by 
patent, these companies have waived all rights and offer no objection to its 
use as recommended by BS i044:Part 1 11964, "Gauge blanks". The prin- 
ciple is illustrated in fig. 3.13, and it is recommended for diameters in excess 
of 60 mm and up to 200 mm. 

At A we see a part sectioned view of the complete gauge. A positive lock 
between gauging member and renewable handle is achieved with the V pro- 
jections shown in the end of the handle at fig. 3.1 3B. These projections locate 
in V grooves machined in the gauging member, which is held in firm contact 
by the screw shown at D. Note that the gauging member is reversible. It is, 
of course, possible to have a double-ended trilock plug gauge, the hollow 
handle permitting the use of a rod to remove the second member. Large- 
diameter plug gauges need protection of the end of the gauging members, 
and this may be achieved by reducing the diameter at the end of the gauging 



72 



WORKSHOP TECHNOLOGY I 



member. This technique is shown at fig. 3.14, together with the use of lighten- 
ing holes ; these holes also serve an additional purpose by allowing the escape 
of air when checking the diameter of a blind hole. 



Gauging member 
reversible 



Screw 



3 V projections 
at I20 6 





Slots for V 
projections 
on both sides. 



End view 
of handle 



Screw 

® 



End view of 
gauging member 



Fig. 3.13 — Trilock Design Plug Gauges. 




Guard 
extension 



Not used 
for blind 
holes 



Lightening 
holes 




For large diameters 

Fig. 3.14. — Details of Gauging Members for Trilock Design. 

3.10 Gap gauges 

We can now turn to the type of gauge required to check the fj shaft on 
which our pulley with its H8 hole is to rotate freely. Always remember that 



INSPECTION 73 

this is the true purpose of limit gauges : to see that the correct type of fit is 
obtained by strict observance of the principle of dimensional control, in other 
words by ensuring that no dimension is outside the limits set by the recom- 
mended tolerances given in BS 4500:1969. 

Three types of gap gauges are mentioned in BS 1044: 
(i) solid gap gauges, type A : flat steel sheet, 
(ii) solid gap gauges, type B : steel forgings, 
(iii) adjustable-type gap gauges. 
All three types are illustrated in fig. 3.15. 



Flat steel sheet 
solid gap gauge 



Steel forged solid gap gauge 






Adjustable gap gauge 

Fi g- 3-!5- — Ga P Gauges. 



There are, of course, variations of the types shown. It is often convenient 
to have a progressive gap gauge ; this is based on the same principle as the 
progressive plug gauge, namely the insertion of the no go member immediat- 
ely after the go member. 

3. 1 1 Ring gauges 

Ring gauges are always used as go gauges ; they check not only diameter, 
but also roundness or concentricity. A typical ring gauge is illustrated in 
fig. 3.16. If the diameter of the shaft to be checked is in excess of 80 mm it is 
common practice to modify the section as shown in fig. 3. 16B, thus providing 
a more positive method of holding and lifting the gauge off a flat surface. 
Fig. 3.16C illustrates a further change in section adopted for the slimmer 
workshop gauges, once again enabling the gauge user to pick the gauge up 
from a flat surface with little risk of dropping and possibly damaging it. 



74 



WORKSHOP TECHNOLOGY I 



3.12 Gauge tolerance 

The manufacture of limit gauges calls for a high degree of skill and experi- 
ence. In the same way that it is not possible for the operator of the machine 
tool to work to precise or exact dimension, it is also not possible for the tool- 
maker or gaugemaker to produce a plug gauge with an exact diameter. 

Let us consider the gauge required for checking the hole in the pulley shown 
in fig. 3. 10. A simple progressive plug gauge is shown in fig. 3. 1 7. As we have 
stated, it is not possible for the tool or gaugemaker to work to the precise 
dimensions shown on the diagram. He must be given a tolerance, and this is 
known as the gauge tolerance. 

C7, 




Alternative 
section 



Fig. 3.16. — Ring Gauge Details. 



3. 1 2. i BS 969:1953, "Plain limit gauges: limits and tolerances" 

This standard is intended to facilitate the manufacture of limit gauges, 
and to ensure that the use of such gauges will not reject any component that is 
within the tolerance laid down. It is the gauge tolerance that leads to diffi- 
culty, for if this tolerance is put outside the component tolerance, the pos- 
sibility will exist of accepting work which is outside the permissible limits. 
Let us have a look at a diagrammatic representation of the effect of the gauge 
tolerance. 

Fig. 3.17B shows such a diagram. It will be seen that the go gauge toler- 
ance is placed inside the work or component tolerance, while the no go 
gauge tolerance is placed outside the component tolerance. The effect of 
placing the go gauge tolerance inside the component tolerance is to reduce 
the amount of tolerance available to the machine-tool operator. It will be 
seen, however, that the gauge tolerance for the no go increases the amount of 
tolerance available. 

Thus the no go gauge will not reject any hole which is within its high limit, 



INSPECTION 



75 



Gaugemakers^ tolerance 
GO AS T NO G O 



04O 
All dimensions in mm 



>- 



r>L 



40039 



® 



I Gauge 
NO GO j/toleronce 




Satisfactory hole rejected 
"* by GO gauge 



Fig. 3.17. — Progressive Plug Gauge Showing Gauge Tolerance. 



® 




NO GO 39-95mm 



GO 39-975 mm 

GO GAUGE 



** 








im^l 


«^ 


V 














f 


c 






t/> 


*»o 






<£ 


o« 






E 


-co 






3 


</)£ 






£ 


3 


I 

.1 




Y//////A 


if 


8 


(D 


NO GO GAU 



Fig. 3.18. — Progressive Gap Gaitge with Gauge Tolerance. 



76 



WORKSHOP TECHNOLOGY I 



40-039 mm, provided that the gauge has been manufactured to the specified 
gauge tolerance. On the other hand it is possible for the go gauge to reject a 
hole which is within the component tolerance. Let us assume that the dia- 
meter of this hole is equivalent to the line XX shown just above the low limit 
of the hole. It will be seen from the diagram that the hole diameter, although 
within the limits allowed, is less than the diameter of the go plug gauge 
when the gauge is made to the high limit of its tolerance. Thus the go gauge 
will not enter the hole, and the pulley is rejected. It is considered, however, 
that such cases are rare events and can be easily resolved by direct measure- 
ment of the component. 

Checking the diameter of the shaft on which the pulley is to be a free rotat- 
ing fit can be achieved by using the progressive gap gauge shown in fig. 3.1 8A. 
Reference back to fig. 3.10 reminds us that the high limit for the shaft is 
39.975 mm, while the low limit is 39.95 mm. At fig. 3.18B we see a diagram- 
matic representation of the gauge tolerance; once again the tolerance of the 
go gauge is within the component tolerance, while the tolerance of the no 
go gauge is outside the component tolerance. 

3.12.2 Obtaining the gauge tolerance 

The recommended tolerances for the gauges described are easily obtained 
from the tables given in BS 969:1953. The amount of gauge tolerance de- 
pends on two factors : 

(i) work tolerance (component tolerance), 

(ii) size of work. 

Table 3.7 shows an extract from the relevant tables entitled "Tolerances 
for ring and gap gauges". It can be seen that the gauge tolerance for the gap 
gauge is 0002 mm. 



I 


2 


3 


4 


5 


6 


7 


Work 

tolerance 

Difference between 

high (H) limit 
and low (L) limit 


Gauge 
tolerance 




Disposition of 


gauge tolerance 




GO 


NOT GO 


H limit 
of gauge 
tolerance 
from H limit 
of work 


L limit 
of gauge 
tolerance 
from H limit 
of work 


H limit 
of gauge 
tolerance 
from L limit 
of work 


L limit 
of gauge 
tolerance 
from L limit 
of work 


Above 

mm 
00075 
0-0175 
0-030 


Up to and 
including 


mm 
0015 
0030 
0034 


mm 
00012 
0002 
00032 


mm 





mm 

— 0001 2 

— 0002 

— 00032 


mm 





mm 

— 0001 2 

— 0002 

— 00032 



Table 3.7 Tolerances for ring and gap gauges 

A similar table gives the recommended tolerances for plug gauges, and 
reference to this table gives the tolerance on the plug gauge as 0-002 mm. 

A further table recommends minimum gauge tolerances relevant to type 
and size of gauge. A cylindrical plug gauge between 25 mm and 50 mm dia- 



INSPECTION 77 

meter has a recommended minimum tolerance of 0002 mm. This agrees 
with the tolerance given to our 40 mm basic-size plug gauge. 

Clearly the tolerances on both the plug gauge and the gap gauge required 
for the checking of our pulley and shaft assembly are very small indeed. The 
accurate determination of the diameters of the gauging members of the plug 
gauge represents a good example of the need for measurement by comparison, 
and it is certain that much use will be made of the comparators described in 
Chapter 2. 

At the same time, determining the widths of the gauging members of this 
gap gauge requires application of the principle of direct measurement; end 
standards or slip gauges will be employed for this purpose. 



Summary 

In this chapter we have seen the interconnecting links between mass- 
production, limit systems, high and low limits on component dimensions, 
limit gauges, and the high and low limits on the gauging members. All this 
stems from the need to assemble mass-produced components that have been 
produced at high production speeds using machine tools. It is not possible to 
machine to exact dimensions; a tolerance must be given to allow for the 
imperfections of the machine tool or operator. Immediately a dimension is 
given a tolerance it has two limits, a high limit and a low limit. Provided the 
actual dimension machined is within these limits it is acceptable, and it is the 
purpose of inspection to ensure that no component having any dimension 
outside the permissible limits is passed to assembly or to the customer. 

The reliability of an engineering assembly is closely connected to the 
accuracy and quality of the different kinds of fits present. These fits may be 
clearance, interference or transition, and the purpose of a limit system is to 
ensure that complete standardisation of these fits is achieved. The limit 
system recommended is the British Standard system, set out in BS 4500: 
1969. Based on the ISO system, BS 4500 offers a comprehensive range of fits. 
The H hole is recommended for engineering assemblies, such holes having 
zero deviation. Thus a shaft with a plus deviation provides an interference 
fit, while a shaft with minus deviation gives a clearance fit. In this way the 
disposition, together with the amount, of the deviation determines the type 
of fit obtained. 

The amount of the tolerance is mainly determined by the class of work or 
the type and quality of the machine tools used. High-grade work such as 
precision lapping is given a small tolerance number, while relatively rough 
work such as sand casting is given a high tolerance number ; thus the tolerance 
number determines the quality of the fit. 

Finally, the limits received by a dimension following tolerancing permit 
the use of limit gauges. Limit gauges provide a rapid and cheap form of 
dimensional control ; they are easily carried about by the inspector, removing 
the need for costly, delicate measuring equipment. Elements of gauge design 
are fully covered in BS 1044 :Part 1 : 1964, while BS 969 : 1953 deals with the 
amount and allocation of the gauge tolerance. 



78 WORKSHOP TECHNOLOGY I 

EXERCISE 3 

i Explain the essential difference between the techniques of measurement and inspection, 
illustrating your answer with a practical example of both techniques. 

2 What is the essential purpose of a limit system? Sketch, using simple engineering 
assemblies, three different types of fit. 

3 Write a simple but adequate description of the main features of the British Standard 
limit system (BS 4500:1969). 

4 Define the following features of BS 4500 : 

(i) ISO tolerances, 
(ii) fundamental deviation, 
(iii) preferred numbers. 

5 What factors determine the choice of the tolerance number when using BS 4500? Why 
are holes usually given a tolerance one grade coarser than the shaft ? 

6 Using the tables provided in this chapter, make a dimensioned sketch of the following : 

(i) A 200 mm pulley with a 50 mm diameter hole to be a close running fit on a 50 mm 

shaft (H7/g6). 
(ii) A 30 mm diameter case-hardened mild-steel pillar to be a drive fit in a mild-steel 
bolster (H7/S6). 

7 Make a neat sketch of an engineering assembly having four different types of fits. Using 
BS 4500 terminology, classify each fit. 

8 What is the purpose of limit gauges? With a neat diagram illustrate the allocation of the 
gauge tolerance, showing clearly how a go gauge may reject a satisfactory component. 

9 Sketch the following gauges : 

(i) double-ended solid plug gauge, 
(ii) progressive renewable-end-type plug gauge. 

10 Sketch the gauges required to check the assembly shown in fig. 3.10 ((£40 H8/f7). 
Insert in full the dimensions of the gauging members, making reference to the tables given 
in 68969:1953. 



4 Cutting Tools 



4.1 Introduction 

We have seen in the previous chapter that if engineering components are to 
be machined for subsequent assembly, the tolerances on the functional 
dimensions may have a very small value. Fig. 4.1 illustrates a phosphor- 
bronze worm wheel casting produced on a capstan lathe. 



Component chucked on this diameter 
55- 




A 


28-3 -28-32 


B 


44-88-44-90 


C 


82-04-8210 


D 


II2-62-I12-69 


a 


LIMITS 



ALL DIMENSIONS IN MM 
PHOSPHOR BRONZE WORMWHEEL 
MACHINING TIME»l-6min 



Fig. 4.1. — Component Machined on Capstan Lathe. 

This casting is held in a three-jaw air chuck and machined with tungsten- 
carbide cutting tools, an overall floor-to-floor time of i-6 minutes each 
being achieved. Note the limits for the bore, namely 28-32 mm high limit and 
28-30 mm low limit, giving the operator a tolerance of 002 mm. The toler- 
ances on the other diameters are also quite small; 0-02 mm on the boss, 0-05 
mm on the recess and 0-07 mm on the outside diameter. Yet this component 
is machined in the remarkable time of 1 -6 minutes, and this includes the time 
required to load and unload the casting. 

In this way a capstan operator is able to machine, say, 230 of these castings 
in a working day, and we can be sure that limit gauges will be used to ensure 
that the diameters are within the limits shown in fig. 4.1. It is worthwhile at 
this point to remember that while gauges will detect any dimension outside 



79 



80 WORKSHOP TECHNOLOGY I 

its limits, preventing the casting being progressed on to the assembly line, 
the fact remains that the casting is still reject, and action must be taken to 
remedy the fault. Although the use of gauges provides a means of dimensional 
control, the accuracy of the machined dimensions is actually determined by 
the efficiency of the cutting tools used. Not only must the cutting tools 
possess the necessary hardness to resist the abrasion inherent in the removal 
of metal but they must also be tough enough to withstand the heavy pressures 
involved when metal is removed at high speed. 

Clearly the cutting tools used to produce the phosphor-bronze component 
shown in fig. 4.1 require not only that the cutting angles are of the correct 
value, but also that the cutting edges are able to maintain their shape and size 
even when the production rate is about 40 components per hour. Remember 
that any wear of the cutting tool used for a specific machining operation must 
result in variation in the size of the machined dimension, together with 
deterioration of the surface finish. Remember, too, that the efficiency of a 
machine tool such as a capstan lathe is measured in terms of the volume of 
metal removal per minute. If we add to this the fact that the machined 
dimensions must be within the prescribed limits, it is clear that the cutting 
tool plays a vital part in engineering manufacture. 

4.2 Cutting-tool materials 

The most important factor in cutting-tool efficiency is the material from 
which the cutting tool is made. Correct design and accurate rake and clear- 
ance angles amount to nothing if the tool material is unable to stand up to the 
cutting conditions. All cutting-tool materials must possess the following two 
properties : 

(i) hardness, 

(ii) toughness. 

Fig. 4.2 illustrates in simple diagrammatic form the properties of hardness 
and toughness with respect to the more commonly used cutting-tool mater- 
ials. Note that the vertical scale represents hardness while the horizontal 
scale shows toughness; it is important, too, to appreciate that the diagram 
gives an indication of these properties at a temperature of 500 C. This is the 
approximate temperature of a metal chip sheared by a cutting tool under 
conditions of efficient machining, although the use of coolants and lubricants 
may reduce this temperature. 

If we now refer to the diagram, taking the various cutting-tool materials 
in turn, we shall see the advantages and limitations of the materials under 
discussion, and we can then turn to the problems involved in the best design 
and application of cutting tools, using the cutting-tool material best suited 
to the particular job in hand. 

4.2.1 High-carbon steel 

Reference to fig. 4.2 shows that high-carbon steel has a relatively low hard- 
ness figure at 500 C. This is owing to the fact that this steel rapidly loses 
hardness when it is taken to temperatures in excess of 220 C; indeed at 
350 C the steel has almost reverted to its normal hardness value. This sets a 



CUTTING TOOLS 



Red 
hardness 




81 



Ceramic 



Cemented 
carbide 

Stcllite 



Toughness 



H.S.S. 



H.C.S. 



Fig. 4.2. — Comparison of Cutting Tool Materials. 

serious limitation on the use of high-carbon steel as a cutting-tool material. It 
cannot be used if high machining speeds are required, such as those used in 
the machining of the wormwheel illustrated in fig. 4.1. It is, however, emin- 
ently suitable for cutting tools not likely to be subjected to undue friction 
leading to a rise in 
temperature, such as 
cold chisels, punches, 
files and scribers. 

Fig. 4.3 shows the 
effect of temperature 
rise on the hardness of 
hardened high-carbon 
steel. It will be seen 
that a rapid fall in 
hardness value takes 
place at about 220 C, 
and at 350 G the steel 
has lost most of the 
hardness gained at the 
initial hardening 
treatment. 



BRINELL 



W^ 5 



800 
700 
600 
Soo 
400 
Soo 
Zoo 
loo 



















































































































































too 2.00 300 400 SOO t 

TEMPERATURE °C 


»0O 


too e 


100 



Fig. 4.3. — Effect of Rise in Temperature on 
Hardened High-carbon Steel. 



4.2.2 High>speed steel 

Fig. 4.4 shows the effect of heat on hardened high-speed steel. It will be 
seen that the hardness increases between 400 C and 550 C. This is quite 



82 



WORKSHOP TECHNOLOGY I 



Brinell 60 °- 
hardness soo- 



Tempering or secondary hardening 



HIGH SPEED 
STEEL 




IOO 200 300400 500 600 XX) 8OQ 

TEMPERATURE C 

Fig. 4.4. — Effect of Rise in Temperature on Hardened High-speed Steel. 

unlike the behaviour of high-carbon steel, and it is clear that the friction in- 
volved when machining at high speeds will not affect the hardness value of 
hardened high-speed steel. The high-speed steel is said to possess the quality 
of red hardness; that is to say it has the ability to retain its hardness value 
even when the swarf is leaving the parent metal at red heat. 

High-speed steel is a relatively expensive metal, easily ten times the cost of 
high-carbon steel. Most drills, reamers and milling cutters are made from 




END MILL 




MILLING CUTTER 



TWIST DRILL 

Fig. 4.5. — High-speed Steel Cutting Tools. 

high-speed steel. Wherever possible welding to a cheaper and tougher 
material will be adopted, providing not only a cheaper tool but also one that 
is more likely to stand up to cutting conditions. Fig. 4.5 shows some typical 
applications of high-speed steel as a cutting-tool material. 

4.2.3 Stellite 

Stellite is the trade name of a special material. It is a non-ferrous alloy con- 



CUTTING TOOLS 83 

taining high proportions of cobalt, chromium and tungsten. Reference 
back to fig. 4.2 shows that Stellite is harder than high-speed steel at a tempera- 
ture of 500 C. When poured in the liquid state from an electric furnace 
Stellite is self-hardening; thus a Stellite casting allowed to cool in air has a 
Rockwell hardness value of about 62 on the C scale. 

This inherent hardening property allows the use of Stellite as a hardfacing 
metal, available for electrodes to be used in the arc-welding technique des- 
cribed in Chapter 1 . Fig. 4.6 illustrates the application of a Stellite electrode 
to recondition or reface a knife-edge bearing surface. It will be seen that a 
layer of Stellite has been welded onto the damaged surface. On cooling the 
Stellite deposit is extremely hard and cannot be machined with normal cut- 
ting tools, but must be ground to the required shape as shown in fig. 4.6C. 



Damaged knife- 
edge 



Stellite electrode 




Re-ground surface 



Fig. 4.6. — Applications of Stellite. 



Fig. 4.6D shows the use of Stellite inserts for a large-diameter milling 
cutter ; this is a much better proposition than making a large-diameter mill- 
ing cutter from high-speed steel. Apart from the high cost of a solid high- 
speed milling cutter, there is the considerable risk of cracking or distortion 
during the heat-treatment process, not to mention the possibility of tooth 
breakage when in use, leading to the scrapping of the cutter. The inserted- 
tooth-type milling cutter using Stellite teeth is an efficient and reliable multi- 
point cutting tool, the high tensile strength of Stellite allowing the inserted 
blades to stand up to the considerable.stress involved when taking heavy cuts. 

4.2.4 Cemented or tungsten carbides 

The exceptional hardness of tungsten carbide was appreciated as far back 
as 1886, but the brittleness of this alloy prevented its use as a cutting-tool 



84 WORKSHOP TECHNOLOGY I 

material. It was the German firm Krupps of Essen who first produced an 
alloy of tungsten carbide and cobalt and successfully used it as a cutting-tool 
material. Called Widia from the German wie Diamant ("like a diamond"), 
it was used as a tip, securely supported and brazed to a carbon-steel shank 
and employed as a lathe tool. It was found that fairly rapid wear of the 
tungsten-carbide tip occurred when the tipped tool was used on the softer 
steels, and this led to the introduction of the elements tantalum and titanium. 
The addition of these elements produces a tungsten-carbide tip with excellent 
steel-cutting properties and a long tool life. 

4.2.5 Ceramics 

Ceramics represent the latest development in cutting-tool materials. It 
will be seen from fig. 4.2 that ceramics possess a higher hardness value than 
tungsten carbide at a temperature of 500 C, but are more brittle. Chemically 
inert, with a low coefficient of friction and low heat conductivity, a ceramic- 
tipped tool has very high metal-removal capacity, together with the ability 
to produce a good finish. Of greater importance is the fact that the life of a 
ceramic-tipped tool is high even when working at maximum speeds, and this 
means that a machine tool such as an automatic lathe equipped with ceramic 
tooling is capable of producing machined surfaces at high production speeds 
to very close dimensional tolerances. 

4.2.6 Application of carbide and ceramic tips 

The inherent brittleness of both carbide and ceramic materials necessitates 
their use as tips for cutting tools. We see on referring back to fig. 4.5 that the 
milling cutters may be made wholly from high-speed steel. This is not possible 
with either carbide or ceramic. These materials are much too brittle, and a 
milling cutter made from either tungsten carbide or ceramic would have as 
much practical value as a delicate china or porcelain ornament. Not only is 
it necessary to use the material as tips ; it is also essential to ensure that the 
tips are given adequate support and are rigidly held in position. 

4.2.7 Tungsten-carbide-tipped tools 

The technique used to join the tungsten-carbide tip to the tool body or 
shank is that of" brazing. A typical tipped lathe tool is shown in fig. 4.7, with 
the machined component shown at B. This tipped tool is used to machine the 
groove to take a V belt. Although several brazing techniques may be em- 
ployed, such as the use of a gas-air blowpipe, electric furnace, gas furnace or 
high-frequency heating coils, essentially the principle remains the same; 
namely the secure joining of the carbide tip while in solid contact with the 
face shown as F on the diagram. It is of the greatest importance that the result- 
ant force exerted on the tip during the cutting operation is taken or resisted, 
not by the brazed joint, but by the solid abutment provided by the face F as 
shown in fig. 4. 7 A. Best results are obtained when using high-frequency 
coils, and this technique is known as induction brazing. Fig. 4.8 illustrates 
both blowpipe and induction brazing. In the case of induction brazing, the 
source of heat is obtained from interference of an intense magnetic field within 



CUTTING TOOLS 



85 




Carbide tip 
to butt against 
solid face F 



lough shank 



Lathe form tool 





DIE CAST ALUMINIUM 
ALLOT V BELT PULLEY 



Fig. 4.7.— Application of Tungsten-carbide Tips. 

the water-cooled copper coil caused by the presence of the tool and tip when 
inserted within the coil. This coil carries a high-frequency current of up to 
500000 hertz. With the tool in position, the operation of brazing is reduced 
to the pressing of a button ; the current is automatically switched on and 
applied for the correct length of time to allow melting of the thin strip of 
copper foil between the tool and the tip. 



Bar to push tip 
vogdnst tool face 



Tool and bit 
inserted in 
coil 





Water cooled 
induction heating > 
coil 



Blowpipe 

Fig. 4.8.— Brazing Techniques for Tipped Tools. 



86 



WORKSHOP TECHNOLOGY I 



Although the tool shank heats at a faster rate than the carbide tip, just 
before the actual brazing temperature the temperature of the tip exceeds 
that of the shank, and this is a desirable feature. 

Although fig. 4.8 shows only a tungsten-carbide-tipped lathe tool, tungsten 
carbide is used to tip drills, reamers and other cutting tools. 

4.2.8 Ceramic-tipped tools 

Although a ceramic tip may be brazed to a carbon-steel lathe-tool shank, 
this technique is seldom adopted when making use of ceramic tips. This is 
owing to the considerable difficulty encountered when attempting to re- 
sharpen a ceramic-tipped lathe tool. It is a better and cheaper proposition 
to discard or throw away the ceramic tip rather than embark on the costly 
and sometimes lengthy project of grinding ceramic tips. 

The adoption of this principle of discarding worn tips has led to the intro- 
duction of the now-popular throw-away tip technique. Fig. 4.9 illustrates a 
typical practical application of this technique. At A we see the principle 
adopted when a square ceramic bit is used to machine a low-strength metal 
such as copper, aluminium alloy or soft steels. These metals require a positive 
rake, and it will be seen from the diagram that this is obtained from the 
design of the tool holder. This means that four cutting edges are available, 
and may be easily and quickly indexed or presented to the work with mini- 
mum loss of time. 

Harder metals require the ceramic tip to have a negative rake, and once 
again the tool holder is used to provide this angle, as shown in fig. 4.9B. 



Clamp 



Tightening Screw 



Chip 
breaker 




Positive rake 
4 cutting edges 



Negative rake 
8 cutting edges 



Triangular 

Fig. 4.9. — Applications of the Throwaway-tip Technique. 



CUTTING TOOLS 87 

The square tip now has eight cutting edges which may be presented to the 
work in turn. 

Ceramic inserts are also available as triangular or circular bits, and fig. 
4.9C shows the application of a triangular bit. It is customary to use a chip- 
breaker behind the ceramic tip. This chipbreaker may be adjustable, as 
shown in the diagram, allowing control of the chip. 

This principle of throw-away tips is now applied to face-milling cutters. 
Three sizes are available, ioo mm, 150 mm and 200 mm diameter; the tips 
used are cemented carbide, locked in place with a simple lever action, with 
provision for roughing and finishing in one pass of the face mill by having 
two opposed finishing tips set lower than the tips used for roughing. 



E 

Q 
LU 
HI 

95 

o 

Z 
H 

o 





H.C. 


High Carbon 


H.S 


High Speed 


ST 


Steilite 


CC 


Cemented carbide 


CER 


Ceramic 



Cutting tool material 

Fig. 4.10. — Cutting Tool Materials and Cutting Speeds. 

4.3 Cutting speeds 

The introduction of the newer cutting materials such as Steilite and 
cemented or tungsten carbides has led to a great increase in the rate of metal 
removal. This is achieved by increase of the cutting speed, that is to say the 
speed of the work relative to the tool when turning on a centre lathe. Fig. 
4. 10 gives some idea of the increase in cutting speed when turning mild steel 
on a centre lathe, and using different tool materials. The shaded areas repre- 
sent the approximate cutting speed in m/s for the cutting tool materials 
shown, while the curved line gives the approximate rev/s when turning a 
mild-steel bar of 25 mm diameter. 

We see at once that a speed of about 5 rev/s is required when using a high- 
carbon-steel cutting tool, while at the other end of the scale it will be seen 
that a speed of about 60 rev/s is required when a ceramic-tipped tool is used. 



88 



WORKSHOP TECHNOLOGY I 



The figures given apply only to a mild-steel bar of 25 mm diameter, as shown 
in the diagram ; smaller diameters or softer metals will require higher speeds. 
It is evident that at the speeds required for carbide or ceramic-tipped tools 
considerable friction must result from the high velocity of the sheared chip 
moving across the tool face or tip face . The velocity of this chip and the pressure 
it exerts on the tool face are the main factors leading to deterioration of the 
tool face and subsequent failure. 

4.4 Tool failure 

Tool failure can be considered as the inability of the cutting tool to maintain 
the required accuracy either of dimensions or surface finish. The period of 
time during which the cutting tool performs satisfactorily is known as the 
life of the tool. A cutting tool that possesses a long life offers very great ad- 
vantages to the production engineer. 

The time required to set, say, a capstan lathe is unproductive time. The 
engineering value of a capstan lathe, as of any other machine tool, lies in its 
ability to produce accurate, well-finished work in the minimum time. Not 
only is the machine idle while the tools are changed ; the cutting tools also 
are expensive, and both time and money are required to restore the cutting 
edges or regrind the cutting tool. The causes underlying tool wear are of 
some importance, for if tool wear could be prevented, in theory a cutting tool 
would last indefinitely, permitting high production figures of well-finished 
components having all dimensions within the limits laid down. 

Fig. 4. 1 1 shows a pictorial view of orthogonal cutting, the condition that 
exists when the tool breast is at 90 to the path of the tool. Note the wedge 



CHIP 
MOVEME 




TOOL MOVEME 
1i 



Depth of cut 

Fig. 4. 1 1 . — Orthogonal Cutting. 



CUTTING TOOLS 89 

shape of the cutting tool. The diagram is representative of shaping, planing, 
milling and turning, or for that matter of all metal cutting using the wedge 
principle. 

Two movements can be seen; the movement or speed of the tool in the 
direction of the arrow shown as B, and the movement or speed of the sheared 
chip in the direction indicated by the arrow A. Thus the tool exerts a force on 
the metal, resulting in partial shear of the metal along the shear plane shown 
as XY. The chip shown is characteristic of mild steel, a fairly ductile metal 
allowing considerable elongation of the chip under the force exerted by the 
cutting tool. 




N « Force normal to tool face 
F m Frictional force of chip 
R • Resultant of N and F 

Fig. 4.12.— Simplified Diagram Illustrating Forces acting at Tool Point. 

Fig. 4.12 shows a side view of the cutting action; the force exerted by the 
tool is at 90 or normal to the tool face, and is shown as JVin the diagram. 

The force exerted by the moving chip is parallel to the tool face, and is shown 
as F. If we now resolve these two forces as shown, then R is the resultant. It 
is this resultant force that leads to tool wear and failure, because of the high 
frictional effect of the fast-moving chip on the tool face. 

4.5 The built-up edge and cratering 

Fig. 4.13 shows the formation of a built-up edge on the tool face. As the 
chip leaves the parent metal the underside of the chip has no time to oxidise, 
and as a result possesses a very clean or pure surface. Forced against the tool 
face by the resultant force R, surface particles of the fast-moving chip weld 
themselves onto the tool face. In this way a build-up of chip metal particles 
takes place, and the moving chip now rides on this built-up edge as it leaves 
the parent metal. The built-up edge is now subjected to the severe frictional 



90 



WORKSHOP TECHNOLOGY I 



force of the moving chip, resulting in severe work hardening and subsequent 
breaking away of metal particles. This formation and disintegration of the 
built-up edge takes place at a very rapid rate, assisted by the high tempera- 
tures and pressures at the tool point. 

As the built-up edge breaks away, minute particles of the tool face, pressure 
welded to the chip, are taken away, and the continuation of this process 
results in the formation of a hollow or crater on the tool face, as shown in fig. 
4.14. The extension of this crater to the point of the tool is the main cause of 
tool failure. 



CHIP FLOW 



Fragments of chip pressure- 
relding to tool face 
forming built-up edge 




Fig. 413- — Formation of the Built-up Edge. 



4.5.1 Avoidance of the built-up edge 

Not only does the formation of a built-up edge bring about eventual failure 
of the cutting tool, but also when components are produced by turning on 
centre, capstan or automatic lathes, small severely work-hardened particles 
of the built-up edge weld or adhere to the surface of the machined com- 
ponents. The presence of these hard particles of metal substantially reduces 
the life of an assembly in which the two mating parts are to be a running or 
rotating fit. 

The following factors tend to delay the formation of a built-up edge : 
(i) high cutting speed, 

(ii) fine feed, 

(iii) large rake angle, 

(iv) sharp cutting edge, 
(v) smooth surface on breast of tool, 

(vi) efficient cutting fluid, 
i) low coefficient of friction between tool and chip. 



vii 



Which of these factors can be introduced and exploited is determined by the 



CUTTING TOOLS gi 

actual cutting tool material and the material being machined. For example, 
if a high-speed steel lathe tool is to be used, then it is important that the rake 
angle be as large as possible without undue weakening of the tool point. The 
smoother the tool face or breast, the more efficient will the tool be, while the 
application of an adequate supply of a suitable cutting fluid will effect even 
further improvement. 

If, too, a roughing tool is used to obtain maximum metal removal with less 
regard for surface finish and dimensional accuracy, then a finishing tool can 
be used with a high cutting speed and fine feed. Carbide and ceramic-tipped 
tools often require no cutting fluids or coolant, and may have a negative rake 
angle because of their relative brittleness. 



CHIP FLOW 



Built-up edge breaking away 
causing crate ring on tool face 



Built-up edge 
re-forming 




Fig. 4.14. — Cratering on Tool Face. 



4.6 Forces acting at cutting-tool points 

The calculation of the forces acting at the point of a lathe tool provides 
an interesting experiment that can be carried out in the workshop or machine- 
tool laboratory. In order to determine the magnitude of the forces acting on 
a lathe tool a dynamometer must be used. A dynamometer is essentially an 
instrument designed to measure energy ; thus a lathe dynamometer measures 
the forces acting on the tool point. 

These three forces are shown in fig. 4. 1 5A, and are the forces exerted by the 
tool on the work. At B we see the equal and opposing forces exerted by the 
work on the tool. These are : 

T = the vertical or tangential force, 

F = the feed force, 

H = the horizontal force. 
The greatest of these three forces is the vertical force T, which can be con- 



92 



WORKSHOP TECHNOLOGY I 



sidered as approximately equivalent to the force present in the turning 
moment of the work. 

Torque = force x radius 
To calculate the power required to machine work on the centre lathe : 
torque (newton metres) = force (newtons) x radius (metres) 
power = 2%NT watts 
where N = rev/s 

T — torque in newton metres 

It is now possible to calculate the following items using the lathe dynamo- 
meter illustrated in fig. 4.16. 

(i) resultant force on tool point and its direction, 

(ii) effect of varying side rakes on power consumed, 

(hi) effect of varying approach angles, 

(iv) machinability of different materials, 

(v) pressure in N/mm 2 on tool point. 

The following notes will give some idea of the use of the dynamometer ; 
the basic principle is shown in diagrammatic form in fig. 4.16. 



® 



Tangential force *T 



Feed force = F 




Forces acting 
on tool 



Fig. 4.15. — Forces Acting on a Lathe Tool. 



4.6. i To find the magnitude and direction of the resultant force on 
a lathe tool 

With the lathe tool securely clamped to the dynamometer with the correct 
length of tool protruding, fix the dynamometer in the tool post. Set all dial 
indicators to zero. Choose suitable rev/s, depth of cut and feed, and note 
readings on all three dial indicators. Convert readings to newtons using 



CUTTING TOOLS 93 

calibration charts supplied with dynamometer; this will give the values of 
forces T, F and H in newtons. 

Draw T and F to a suitable scale and at right angles. As shown in the 
diagram (fig. 4. 16A) the resultant of these two forces (RFT) is at an angle of 
a° to the vertical plane; this angle may be carefully measured. We must now 
consider the effect of the horizontal H, and construct another vector 
diagram with the object of resolving the two force components RFT and H. 
This is also shown in fig. 4.16, and the resultant of these two components is 
indicated as R. Note that it makes an angle of 0° to the horizontal. 

The angles can if required be calculated mathematically, and the value of 
this experiment lies in the fact that it is possible to calculate the resultant 
thrust on a carbide or ceramic tip, and design the tool holder so that this 
thrust is opposed by a machined face. 

4*6.2 To find the effect of varying top rakes 

Four knife-edge lathe tools are required with the following side rakes: o°, 
io°, 20 and 30 . 

Set each tool in turn in the dynamometer and note the dial indicator read- 
ings for the vertical force T. Convert readings to newtons and hence to 
watts; then plot power against side rake angle, as shown in fig. 4.16B. This 
experiment is most useful in proving that increase in rake angle decreases 
the shear plane and thus the energy requirements. 

4.6.3 To determine the machinability of different materials 

Several bars of identical diameter but of different materials are required 
for this experiment. Suggested materials are mild steel, brass, copper, grey 
cast iron, duralumin, perspex and nylon. 

A suitable (though approximate) technique is to keep the tool angles, 
rev/s, feed and depth of cut constant for each machining test; thus the only 
variable is the metal or material under test. The magnitude of the vertical 
force 7" may be taken as an indication of the machinability of the material. 
As mild steel is perhaps the most widely machined metal it can be given an 
index of 1, and relative machinability figures can be obtained in proportion 
to the vertical force T obtained in each case. Fig. 4. 16C shows a typical chart 
giving an indication of the machinability of the more common engineering 
materials. 

4.6.4 To find the pressure acting on a lathe tool 

This experiment is best carried out using a mild-steel test piece. With a 
reasonable depth of cut, feed, and rev/s, note the reading on the dial in- 
dicator for the vertical force; convert into newtons using the calibrated 
chart. 

This force acts on an area equal to the feed multiplied by the depth of cut, 
as shown in the diagram (fig. 4.16D). Let us take some approximate figures. 
Let depth of cut = 2mm 

feed = 02 mm 
force T = 650 N 



94 



RFT 



WORKSHOP TECHNOLOGY I 

JLLnO«002mm Dial indicator 




Plan view of 



RFTE 



Htiampad in tooipost lathe dynamomgter 



» —1 


1 













































































































































RAKE ANGLE 




IO 



d- depth of cut < 
f =feed/rev 



Ik 



5 



MATERIAL 



Fig. 4.16. — Practical Applications of the Lathe Dynamometer. 

Area of chip = 2x0-2 = 0-4 mm 2 

On an area of -fa mm 2 acts 650 N 

On an area of 1 mm 2 acts 

6 5° XI ° N= 1625N 
4 



CUTTING TOOLS 9,5 

Now a force of 1 625 N/mm 2 is equivalent to 1 625 MN/m 2 , and as the shear 
stress of mild steel is about 400 MN/m 2 it is clear that this value greatly exceeds 
the shear strength of mild steel, and accounts for the apparent ease with which 
the chip leaves the parent metal. 

4.7 Radial cutting 

Fig. 4. 15 illustrates a typical example of radial cutting, that is to say cut- 
ting with the tool in line with the radius of the turned work. In view of the 
relatively severe pressure acting on the cutting tool, usually in the range of 
900 to 1600 N/mm 2 for mild steel, this is not the best way of presenting the 
tool to the work. 

Fig. 4.17 illustrates the principle involved. The vertical force F acting 
downwards induces bending and vibration of the cutting tool with subsequent 
deterioration of the dimensional accuracy and finish of the turned work. 
This effect is often known as chatter, and if the work/tool area of contact is 
large, the machine tool is likely to vibrate in harmony with the cutting tool, 
setting up a noisy clangour. 




Tangential 
Cutting 



Form tool 



Fig. 4.17.— Radial and Tangential Cutting. 
4.8 Tangential cutting 

The principle of tangential cutting is shown in fig. 4.17B. It will be seen 
that the tool now lies along a plane tangential to the surface of the turned 
work. In this position the tool is better able to absorb the force exerted on it, 
and the possibility of deflection or vibration is reduced to a minimum. A 
popular application of tangential turning is the use of form tools. Inevitably 
a form tool must have a relatively large work/tool contact area, and much 
greater rigidity is obtained if the principle of tangential cutting is adopted. 
Thus the form tool illustrated at fig. 4. 1 7C is applied to the work as shown at 
B, and locates in a dovetail slide. 



96 WORKSHOP TECHNOLOGY I 

Another popular application of the tangential cutting principle is the 
chipstream roller box used for reduction of diameters on the capstan lathe. 

4.9 Negative-rake cutting 

Negative-rake cutting, as the name suggests, involves the use of cutting 
tools having a negative rake. This technique is applied only when relatively 
brittle cutting materials are used. The reason for this is that the grinding of a 
positive rake on a cutting tool, although reducing the area of metal to be 
sheared, reduces also the volume of metal at the point of the cutting tool it- 
self, and this is precisely where the maximum amount of metal is needed. 

Tool point weakened 

® 





Unsupported 



Supported 

With XY horizontal the principle can be 
applied to horizontal milling 

Fig. 4.18.— Positive and Negative Rake Cutting. 

This is shown in fig. 4.18A, where it can be seen that the rake angle has 
considerably reduced the ability of the tool point to withstand not only the 
considerable pressure exerted by the cutting or shearing action, but also any 
sudden blow or shock encountered when the tool first makes contact with the 
work. If we consider the application of cemented-carbide and ceramic tips, 
with their excellent hardness but inherent brittleness, then it is clear that if 
these tipped tools are provided with positive rake their life is likely to be very 
short. The smallest amount of chipping or damage on the breast of these 
tipped tools has a marked effect on the ability of the tip to remove metal 
efficiently, and it is essential that the tip be provided with the maximum 
strength in order to promote a long life. This is achieved by providing the 
tipped tool with negative rake. 

Fig. 4. 1 8B shows the side elevation of a negative-rake tipped tool. It can be 
seen that the vertical cutting force is taken on a supported section of the tool, 
and not on the unsupported section of the tool point, as shown at A. The 
increased strength of the tool point obtained by adopting the negative-rake 



CUTTING TOOLS 97 

principle allows the use of very high cutting speeds, as we have seen in fig. 
4.10. For example, when cutting mild steel at a cutting speed of approxim- 
ately 4 m/s, a 25 mm diameter bar will require a spindle speed of about 50 
rev/s, if it is being machined on a lathe. 

A considerable amount of heat is generated at the shear plane, raising the 
temperature of the metal to such a degree that the chip becomes semi-plastic 
and less force is required to shear it from the parent metal. In this way the 
increased shear plane caused by the negative rake is counteracted by the 
greater ease with which the plastic chip is sheared from the parent metal. 
It must now be appreciated that the correct application of both cemented- 
carbide- and ceramic-tipped tools demands the use of machine tools specially 
built for carbide or ceramic tooling. 

We may list the essential requirements for negative-rake cutting as follows : 
(i) maximum rigidity of machine tool, work and cutting tool, 
(ii) provision for ample range of high cutting speeds, 

(iii) adequate power supply, 

(iv) avoidance of any rubbing of tool, 
(v) mirror finish on breast of cutting tool. 

4.10 Cutter grinding 

The renovation of the cutting edge of a Cutting tool is still an essential 
requirement. Although the use of throw-away ceramic tips removes the 
necessity for cutter or tip grinding, we must remember that the use of such 
tips is confined to machine tools especially designed for this type of tooling, 
and this means that a very great percentage of metal removal is still achieved 
with the use of high-speed-steel cutting tools. 

Apart from correct maintenance of the rake and clearance angles, it is an 
essential part of cutter grinding that the best possible finish be obtained on 
the tool breast. With carbide or ceramic tips a final lapping operation is 
needed, using diamond-impregnated lapping discs or Bakelite-bonded 
lapping wheels impregnated with diamond dust. 

The mirror finish possible when lapping the tipped cutting tool materially 
reduces the friction between the sliding chip and the tool breast or face. In this 
way the formation of the built-up edge is prevented, and there is little doubt 
that the operation of lapping a mirror finish to the tip breast is worthwhile, 
resulting in considerable increase in cutting-tool life. 

Summary 
Cutting tools are still a vital and essential part of modern engineering 
manufacture. We have seen that the advance in cutting efficiency has re- 
sulted as a direct outcome of the introduction of new cutting-tool materials. 
The main property required of a cutting-tool material is that it prevents the 
formation of a built-up edge and consequent cratering. Both cemented car- 
bide and ceramic materials are used as tipping media for cutting tools, while 
the latter are also finding increasing application for throw-away tips. At the 
same time there is still an important use for both high-carbon and high-speed 
steel cutting tools. Provided a tool is not likely to be subject to an appreciable 



98 WORKSHOP TECHNOLOGY I 

rise in temperature when in use, high-carbon steel is ideal, and most hand 
tools are still made from this material. 

High-speed steel is still in widespread use for milling cutters, drills, reamers, 
and many other cutting tools used on fairly modern machine tools. In all these 
applications the correct rake angle is of the greatest importance, reducing the 
energy requirements and promoting a good finish. The use of the negative- 
rake technique is restricted to carbide or ceramic-tipped tools, the tool 
points gaining considerable additional strength. 

Finally it must be remembered that carbide and ceramic tooling demands 
the use of machine tools built for this type of machining. Ample horsepower, 
great rigidity and a high range of cutting speeds are essential requirements 
if the best use is to be obtained from the new cutting materials now available. 

EXERCISE 4 

i Explain why high-carbon steel is seldom used as a material for making milling cutters. 
Sketch three cutting tools which would be quite serviceable when made from high-carbon 
steel. 

2 Define the term "red hardness". Sketch a cutting tool that can be used for high speed 
turning at a centre lathe, stating the material from which the tool is made. 

3 What advantages are offered by the use of Stellite as a cutting-tool material? Give a parti- 
cular use for this material. 

4 Why are the cemented carbides always used as a material for tipping to* Is? Make a neat 
sketch of a carbide-tipped lathe tool. 

5 Sketch a typical throw-away-tip ceramic lathe tool. 

6 What is meant by the term "built-up" edge? Outline the factors that promote the 
formation of a built-up edge. 

7 Why is it essential to reduce or prevent the formation of a built-up edge? 

8 Describe a typical experiment that could be carried out to determine the effect of rake 
angle on high-speed steel lathe tools on : 

(i) power consumed, 
(ii) pressure on tool point. 

9 With the aid of a neat sketch show the essential difference between tangential and radial 
cutting with respect to lathe tools. 

io Explain why cemented-carbide and ceramic-tipped tools are usually given negative rake. 
What are the essential factors underlying the correct use of negative-rake tipped tools? 



The Centre Lathe 

5.1 Introduction 

We are concerned in this chapter with further work on the centre lathe. It 
must be appreciated at the outset that the use of a centre lathe demands a 
highly skilled and experienced craftsman, but most of the techniques he 
employs are also basic to the use of capstan, turret and automatic lathes. As 
with all metal-removing machine tools, the centre lathe has as its primary 
purpose the production of geometrical surfaces. There are few geometrical 
surfaces that cannot be produced at the centre lathe, and for this reason it is 
an essential machine tool, always found in toolrooms, maintenance shops, 
development and prototype departments, and in most other places where 
the business of engineering manufacture is carried on. 

5.2 Limitations of the centre lathe 

Although capable of producing a wide range of surfaces, including cylin- 
drical, plane, conical, spherical and helical, the centre lathe suffers from the 
disadvantage that the tool-holding and work-holding devices used require 



3 Jaw 
chuck 



4 Jaw 
chuck 



Collets Face plate Centres 




Headstock 
(work-holding^ 



Drills Reamers 
Tailstock 
tool - holding 
work-holding 



4 Way tool post 

cutting tools 
knurling tools 
form tools 
boring tools 

Fig. 5.1. — Work Holding and Tool Holding on a Centre Lathe. 



99 



IOO WORKSHOP TECHNOLOGY I 

considerable setting and changing. Fig. 5.1 shows the plan view of a centre 
lathe, and indicates the tooling positions and work-holding stations. It will 
be seen that the work-holding devices include chucks, collets and faceplate, 
all of which must be attached to the lathe spindle. 

5.3 Tool holding 

The tool-holding device on a centre lathe is the four- way tool post, which 
can be indexed to the workpiece, while further tools can be held in the tail- 
stock. Thus a centre lathe equipped with a four-way tool post and a tailstock 
can be said to have five tooling stations and one work-holding station. This 
means that if the centre-lathe turner wishes to use an alternative method of 
work holding it will be necessary to remove the work-holding device already 
in the lathe and replace it with the alternative device. Each time a different 
drill or reamer is used in the tailstock it means that work stops while the tools 
are changed. 

It is perhaps not realised that the skilled turner spends a great deal of his 
time, not in actual machining, but in tool changing and setting. It may be 
said, too, that this tool changing and setting represents the more skilled side 
of his work; the actual removal of the metal and the geometrical accuracy of 
the machined surfaces is the province of the machine tool, although linear 
and angular accuracy are under the control of the centre-lathe turner. We 
will see in the next chapter the great advantages offered by capstan and turret 
lathes with regard to this matter of tool changing and setting; there the skill 
of the craftsman is applied to the tooling of the lathe, thus permitting the use 
of semi-skilled personnel for the actual machining or operating of the machine 
tool. 

5.4 Work holding 

Let us now take a closer look at the use of the work-holding devices avail- 
able on a centre lathe, keeping in mind the fact that the same principles will 
be used when we deal with capstan and turret lathes. 

5.4.1 Chucks 

A lathe chuck may be considered as the equivalent of a bench vice, except 
that while the vice is permanently attached to the work bench, the lathe 
chuck must be capable of accurate and temporary location to the lathe 
spindle. Like a vice, a chuck possesses gripping jaws, and lathe chucks are 
described according to the number and type of jaws. Both three- and four- 
jaw chucks should be familiar at this stage of our studies, including the draw- 
in-collet type of chuck. All these chucks are used for the holding of circular 
work, although the standard three-jaw chuck is equally capable of holding 
hexagonal work. Although seldom used, two-jaw chucks are available, 
having soft mild-steel jaws which can be machined to accommodate a cast- 
ing or forging of irregular shape. Fig. 5 .2 shows the basic uses of the more com- 
mon types of chucks, and their respective applications are outlined below. 

Fig. 5.2,4 

Three-jaw universal chuck, self centring ; all jaws move together, with the 



THE CENTRE LATHE 



101 



work held on either inside or outside faces of jaws. Two sets of jaws available ; 
the reversible jaws considerably increase the effective work-holding dia- 
meter. Much used for the production of small parts in one setting, e.g. drill 
bushes from cold rolled or bright drawn sections. 

Fig. 5.25 

Four-jaw independent chuck; all jaws have independent movement. 
Superior gripping power to three-jaw chuck. Used for holding square, 
octagonal or irregular work. Often used for second settings in conjunction 
with dial indicator to obtain concentricity with previous setting. Unlike the 
three-jaw chuck, in which the chuck centre line is that of the lathe, a four- 
jaw chuck has a variable centre line, determined by the positions of the four 
jaws. This difference between the lathe and chuck centre line allows the turn- 
ing of diameters which are eccentric, or not concentric with each other. 

Component machined 
to one setting 




Large components 
Irregular components 
Second settings (& 



„ Collet 



t 



2-jaw 




Bars of standard 
section fQ 



Castings and forgings 

® 



Fig. 5.2. — Work -holding Devices. 

Fig. 5.2C 

Draw-in-collet chuck, more commonly referred to simply as a collet. 
Although it is not immediately apparent, collet chucks have three jaws which 
close and grip the work when the collet is pulled or drawn into its seating. 
Most accurate of all the chuck family, collets are mainly used for the holding 
of standard diameters, although collets for square or hexagonal sections are 
available. It must be appreciated that a collet is usually only suitable for one 
specific diameter or size, and this means that a range of collets is required. 

Fig. 5.2Z) _ 

Two-jaw chuck; more commonly used on capstan or turret lathes, but 



102 



WORKSHOP TECHNOLOGY I 



most useful when dealing with prototype castings to be machined on centre 
lathes. The jaws are of soft mild steel, capable of being case-hardened. The 
profile of the casting is machined in the two soft jaws, allowing the gripping 
or holding of a casting that could not be held in any other manner. Forgings 
may also be held in suitably machined two-jaw chucks. 

5.4.2 Faceplate 

The purpose of a lathe faceplate is to provide a datum face or reference 
plane at 90 to the lathe centre line. Initially this datum face permits the 
location and clamping of castings or other components having relatively 
large surface areas, but it is possible also to clamp additional equipment such 
as angle plates and V blocks to a lathe faceplate. 



50 ±0025 



ALL DIMENSIONS IN MM 




3RD ANGLE 



Tightening . 
screw 



Marked out 
drilled and 
tapped 




Fig- 5-3- — Application of Toolmaker's Buttons. 



Fig. 5.3 illustrates a typical component requiring the use of a faceplate; it 
is required to bore the 50 mm diameter hole shown to within a tolerance of 
plus and minus 0-025 millimetres from the faces shown as X and Y in the 
diagram. Note also the two 25 mm diameter holes at a centre distance of 
62 mm. We will see that it is the setting up that requires the skill, while the 
machining is a relatively simple affair. 

Toolmaker's buttons 

The use of toolmaker's buttons in conjunction with the faceplate of a 
centre lathe is an interesting example of the use of end standards and reference 
planes allied with turning techniques. Three buttons are required for the job 
shown in fig. 5.3, and the purpose of these buttons is to ensure that the linear 



THE CENTRE LATHE 



103 



dimensions are within the limits shown on the diagram, namely plus and 
minus 0-025 millimetres. Fig. 5.3B shows the preliminary operation of 
marking off the dimensions; drilling and tapping the three hole centres for 
the button-holding studs. The dimensional accuracy of the hole centres from 
the datum faces is not, at this stage, of great importance. 

A typical button is shown at fig. 5.3C; note that considerable clearance 
exists between the diameter of the stud and the internal diameter of the 
button. Reference to fig. 5.4 will make clear the necessity for this clearance. 
Here we see the method of ensuring that the linear dimensions of the hole 
centres with respect to both centre distances and distances from the datum 
faces are held to very close limits. The 50 mm diameter hole is first dealt with. 

The toolmaker's button is first positioned by lightly screwing down the 
nut; alternatively a long screw may be used. Toolmaker's buttons are hard- 
ened cylindrical steel bushes, available in sets of three or more, and of differ- 
ent lengths, but with outside diameters identical and to a specified size, say 
20 mm. 

Thus the distance from the face X to the centre of the toolmaker's button 

must be 50 mm minus one-half the button diameter : 

20 
50 = 40 mm 

In the same way the distance of the button centre from the datum face Y will 

be: 

200—10 = 190 mm 

The clearance of the stud or screw now permits adjustment of the button 
with the slip-gauge pile (190 mm) in the position shown in fig. 5.4A. When the 



Face X 




Face Y 



Section through 
^d\ button 

Fig. 5.4 — Method of Setting First Button. 



io4 



WORKSHOP TECHNOLOGY I 



feel of the slip pile between the button and the surface plate is to the satis- 
faction of the turner, the button is securely tightened in place and then care- 
fully rechecked to ensure that no subsequent movement has occurred. 

At fig. 5.4B is shown a sectional view of the button; it will be seen that an 
essential condition of accuracy is that the button faces be truly at 90 to the 
outside surface. 

With the ultimate position for the 50 mm diameter hole assured by the 
precise location of the button, the two remaining holes may now be dealt with 
in a similar manner, as shown in fig. 5.5. 

ALL DIMENSIONS IN MM 
20 Buttons 




Button C clocked to same height as button B 

Fig. 5.5. — Toolmaker's Buttons in Position. 
Boring the holes 

The workpiece, with the three toolmaker's buttons securely attached, 
must now be clamped to the faceplate, and if the large-diameter hole is the 
first to be bored the centre of the button must lie on the centre line of the lathe. 
In other words the external surface of the button becomes a datum face and 
can be used to bring the centre of the button on the lathe centre line. This is 
done by using a dial indicator or clocking the button ; the technique is shown 
in fig. 5,6. In this way the axis of the button coincides with the axis of rotation, 
provided the dial indicator pointer reads zero through one complete revolu- 
tion of the work. 

The button is now removed, the threaded hole opened out to within about 
0-5 mm of finished size, a boring bar mounted in the tool post, and the hole 
bored to the finished size. The same procedure is repeated for each hole in 
turn, and provided the necessary care has been taken with respect to final 
checking of the buttons and clocking prior to drilling and boring, no trouble 



THE CENTRE LATHE 



IO5 



Dial indicator 
attached to lathe 
toolpost 



T slots ft clamping arrangements not shown 



Parallel bars 




FACEPLATE 



Tool makers button 
athe centre line 



Button centre line 

Fig. 5.6. — Method of Clocking Button. 

will be experienced in keeping the linear dimensions to within plus and 
minus 0-025 millimetres. 

Perhaps this example of the use of a lathe faceplate, together with the 
application of end standards or slip gauges, will serve to demonstrate not only 
the considerable skill required by the centre-lathe turner, but also the large 
amount of time occupied by setting up for the machining operation. If a large 

-Workplace 




Component with holes^ 
close together 

Fig. 5.7.— Technique for Close Proximity Holes. 



io6 



WORKSHOP TECHNOLOGY I 



number of holes were required to close limits of accuracy, then the time 
required to set up the work using toolmaker's buttons would be quite out of 
proportion to the machining time. To meet the need for accurate hole drill- 
ing in the minimum of time, machine tools called jig borers have been 
introduced. 

Before leaving the subject of button boring it must be mentioned that when 
holes are close together considerable difficulty may be experienced when 
attempting to clock the buttons. To overcome this the buttons are available 
in different lengths, or alternatively they may have the section shown in fig. 
5.7 ; note the close proximity of the two holes. The clocking of the buttons is 
now achieved by locating the plunger of the dial indicator on the internal 
cylindrical surface of the button, which is of course concentric with the out- 
side diameter. 

Faceplate balancing 

Reference back to the set-up illustrated in fig. 5.6 shows that the faceplate 
carries a considerable mass of equipment ; in addition to the component 
and parallel bars shown, it is also necessary to make use of suitable clamps 
and clamping bolts. Fig. 5.8 shows a plan view of the faceplate with the set- 
up for button boring. Note that the faceplate is out of balance owing to the 
fact that the mass of the parts used in the set-up is not distributed evenly about 
the lathe centre line. 




OUT OF BALANCE FACEPLATE 



Fig. 5.8. — Principle underlying Faceplate Balancing. 

Rotation of the faceplate under these conditions of unequal balancing is 
not only damaging to the spindle bearings but also dangerous to the operator. 
Added to these most undesirable features we have the vibration caused, which 
will be transmitted to the surface of the bored holes, resulting in a poor finish. 
The use of balancing masses eliminates all the unwanted characteristics of 
an unbalanced faceplate ; the technique is shown in fig. 5.8. With the spindle 
set in the neutral position so that it rotates freely, masses are either added to 



THE CENTRE LATHE IO7 

or taken from the positions shown at X and Y until the faceplate has the tend- 
ency to stop in different positions after being given a swing by hand. 

If the faceplate comes to rest in the same position after repeated swings it 
is a sure indication that a condition of unbalance still exists. It is possible, 
though very seldom done in practice, to calculate the amount and position of 
the balancing masses required to balance a set-up or a casting of which the 
mass and centre of gravity are both known. 

Thus fig. 5.8A shows the out-of-balance effect as being equal to Mxr. 
Provided the value M is known in kilograms it is a relatively simple matter 
to calculate either of two things : 

(i) the distance from the centre at which to put a balancing mass of 

known value, 
(ii) the amount of the balancing mass to be put at a given centre distance. 
With the balancing mass placed directly opposite to the out-of-balance 
force as shown in the diagram, the following formula may be used : 

Mxr = M x x r x 
With M, r, and M x known it is a simple matter to calculate the distance 
from the centre at which to put the known balancing mass. Alternatively the 
method shown at fig. 5.8B may be adopted if two balance masses are to be 
used. Let M represent the out-of-balance force acting at a distance R from 
the faceplate centre. It is required to balance this force by clamping two 
masses as shown in the diagram; the angle between the two masses is 90 , 
as can be seen. The following information is known : 

Mass of set-up acting at its centre of gravity M = 30 kg 
Distance of M from faceplate centre = 280 mm 

Mass of balance mass M t = 30 kg 

Mass of balance mass M 2 = 35 kg 

It is required to calculate the distances at which to clamp the balance 
masses M x and M 2 , for with these distances known the time required to 
balance the faceplate is very much reduced. 

Use of a vector diagram 

Vector diagrams are an application of scientific principles. Their practical 
value may be appreciated if we consider- the problems encountered when 
attempting to balance a lathe faceplate. 

The out-of-balance force caused by a rotating set-up similar to that shown 
in fig. 5.8 is proportional to the mass and its distance from the lathe centre 
line, the out-of-balance force acting outwards from the centre. A simple force 
diagram is shown in fig. 5 .8C ; note that the three forces are in equilibrium and 
that they are represented by product of the mass and its distance from the 
centre. 

A vector diagram may now be constructed by drawing to a suitable scale 
three lines representing the direction and magnitude of each product of mass 
and distance. This diagram is illustrated in fig. 5.9. The vector ab is drawn at 
30 to the horizontal and to a scale of (say) 1 : 100. This vector represents the 
out-of-balance force of the casting, which is equivalent to Mr or 30 x 280 = 
8400 kg mm. 



io8 



WORKSHOP TECHNOLOGY I 



Referring back to fig. 5.8B the reader will see that a balancing mass is 
shown in the vertical position, or at an angle of 120 to the centre of gravity 
of the casting. The out-of-balance effect produced by this 30 kg mass {M x ) 
is represented vectorially by drawing a vertical line through b, as shown 
in the diagram. 

A similar procedure is adopted for the second balancing mass of 35 kg 
(M 2 ). As can be seen, this weight is at 90 to M u and can be represented 
vectorially by drawing a horizontal line through b in the vector diagram, 
meeting the previously drawn vector at c. This completes the construction of 
our vector diagranj.. 

It is now possible, with the aid of this diagram, to calculate the distances 
at which to place the balancing masses M x and M 2 . Careful measurement of 
both vectors and conversion to actual units gives the out-of-balance effect in 
each case. Thus for the mass M i the out-of-balance effect is 42 x 1 00 = 4200 



M2R2 



73mm 



42 mm 




30 kg x 280mm 
= 8 400 kg mm 



84mm 



Scale: IOO 



Fig. 5.9. — Vector Diagram. 



kg mm, because on measurement be is found to be 42 mm. Similarly, for M 2 
it is 73 x 100 = 7300 kg mm, since ca measures 73 mm. 
Now we know that 



so that 
and 
so that 



M x x R x = 4200 

R, = 
M 2 xR 2 = 7300 



4200 

— — = 140 mm 

30 * 



R 2 = — — = 2083 mm 

2 35 J 



The balancing masses are now clamped as follows : 

the 30 kg mass at a radius of 140 mm, 

the 35 kg mass at a radius of 208-3 mm. 
Reference back to fig. 5.8B will show the positions for these two masses. 
Remember that the mass of the casting to be balanced acts at the centre of 
gravity of the casting. 



THE CENTRE LATHE 



IOg 



5.4.3 Centres 

We have seen that the use of a faceplate as a work-holding device involves 
the centre-lathe turner in considerable setting of the work to be turned. This 
is true also for the use of a four-jaw chuck, and if the turned work is to be 
passed on for further machining (such as milling or grinding) it is certain that 
centres will be used as a work-holding device. Not only do the centres offer 
a means of holding the work, but they also provide datum or location points, 
permitting the rapid setting up of work to be turned. A typical component 
requiring the use of centres is shown in fig. 5. 10 A. This is a splined gear shaft 
with turned diameters, milled splines and milled gear teeth. For each of the 
machining operations given above, the centres will provide the location and 
work-holding points; in this way concentricity of the turned diameters, the 
splines and the gear teeth is assured. 

Because the centres are the datum points or faces it is essential that they be 
carefully machined, and fig. 5.10B shows a simple sectional view of one end 
of the splined shaft. Note the small recessed diameter; this helps to prevent 
damage to the bearing surfaces of the centre. A good finish is required on the 
bearing surface shown as aa in fig. 5. 10C. When the centre is being machined 
an excellent finish will result if the centre drill is allowed to dwell after the 
drill is taken to depth, with an adequate supply of cutting fluid supplied to the 
drill point. Should the component be case-hardened it is a good plan to lap 
the centres carefully, using a short length of copper rod. One end of this rod 
may be turned to 6o°, as shown in fig. 5. 1 1 A ; a small drilling machine is quite 
suitable for the lapping operation, and a smooth-grade lapping compound 
is needed. The technique is illustrated in fig. 5.1 iB, and is to be carried out 
after the case-hardening of the turned blank. 

If the part cannot be easily supported on the drilling-machine table it is a 




Small recess to prevent damage 



Fig. 5.10.— Application and Details of Centres. 



no 



WORKSHOP TECHNOLOGY I 



simple matter to hold the copper lap in a suitable collet inserted in the spindle 
of a centre lathe. With the other end of the component located in the tailstock 
centre, the part may be held in the hand and gently brought up to the revolv- 
ing lap, the movement being brought about by rotation of the tailstock hand 
wheel. This technique is illustrated in fig. 5. 1 1 C. 

It will be clear from the example given that the use of centres is limited to 
components of solid section, although special devices are available for the 
support of components having drilled or bored holes. When the hole or bore 
is large enough, the principle of turning between centres is extended by the 
use of a suitable mandrel. 



chuck 




Component held 
by hand 

Fig. 5.1 1. — Methods of Lapping Centres. 

5.4.4 Mandrels 

Mandrels are work-holding devices having the additional advantage of 
precise location ; this is provided by accurate centres at both ends. Their main 
use is to allow the machining of outside diameters or the cutting of threads 
concentric with a previously bored hole. All mandrels should be made from 
tool steel suitably hardened and tempered, although there is no objection to 
the use of a case-hardening mild steel. Several types of mandrels are available, 
depending on the method adopted to hold or grip the workpiece. Some simple 
types are illustrated in fig. 5.12, together with typical applications. Remem- 
ber that a mandrel is a precision work-holding device and must be treated as 
such. The bearing surface must be free from damage or burrs, while the 
centres must be kept clean and in good condition. The use of a friction man- 
drel demands considerable care. If the fit is too tight and undue force is used 
to assemble the component, scoring and misalignment of the bore are likely 



THE CENTRE LATHE 

To per OOlmm per 25mm 



ii i 



m 



m 




Standard mandrel 

Turning outside diameter concentric with bore 



m 



E 




Threaded gang mandrel 

Several diameters at the same time 





Facing threaded 
component 



Threaded mandrel 



Fig. 5.12. — Applications of Mandrels. 

to result, with the attendant possibility of rejection of the finished com- 
ponent. The application of a little oil to the surface of the mandrel is always 
a wise precaution. 

5.5 Screw cutting 
The cutting of screw threads is an accepted part of the work of a centre- 
lathe turner. All modern lathes are fitted with gearboxes, allowing a wide and 



— Hand lever 



3 start— 
Acme thread 

45mm lead 




ISO -- -—"^ 

coarse x2-5 
mm 



1 


1 



Fig. 5.13.— Fly Press and Fly Press Leadscrew. 



112 



WORKSHOP TECHNOLOGY I 



instant choice of both English and metric pitches to be selected merely by the 
movement of gear handles. It may be, however, that an awkward pitch or 
multi-start thread may require cutting, and the following example will 
serve to illustrate the considerable knowledge and skill involved. 

Fig. 5.13 shows the screw of a small fly press; the principle of the fly press 
or hand press is also shown in fig. 5.13, and it will be seen that rotation of the 
hand lever by the operator provides vertical movement of the ram. This type 
of press is much used for small-capacity work, and it is important that a con- 
siderable vertical movement of the ram results from each revolution of the 
hand lever if the operator is to be relieved from undue effort. This means that 
a large or coarse-pitch thread is required, and as the thread is required to 
transmit motion, a square or Acme form is essential. Note that a single-start 
thread of ISO coarse form and having a pitch of 25 mm is also required ; 
the purpose of this thread is to allow secure clamping of the stop collar, thus 
controlling the setting of the punch for depth. 

5.5.1 The need for a multi-start thread 

We have seen that a coarse thread pitch is essential if the time and energy 
to be expended by the operator are to be kept to a minimum. It is important 
to appreciate that the depth of thread is proportionate to the pitch ; the coarser 



0-37P 



ALL DIMENSIONS IN MM 
ACME FORM. 




OD- Outside diameter - C+2 D - 113-75 
C=Core -3D -68-25 
D -Depth of thread - 22-75 

Fig. 5.14. — Single-start Thread. 

the pitch, the deeper will be the thread depth. With a single-start thread this 
would tend to produce severe weakening of the screw, which can be avoided 
only by increase in diameter. Fig. 5.14 shows the effect of cutting a single- 
start thread of 45 mm pitch, which is the pitch required for the screw of the 



THE CENTRE LATHE 



113 



fly press. It will be seen that the thread proportions for an Acme thread are 
such that the depth of thread is equal to one-half the pitch plus 0-25 mm. 

At. fig. 5 . 1 4B we have made the core diameter equivalent to three times the 
depth of thread in order to produce a screw of reasonable strength. Reference 
to the calculations given makes it clear that the outside diameter of the screw 
will be 1 1 3-75 mm. Clearly this diameter is out of all proportion to the applica- 
tion of this screw to a simple fly press ; yet if a single-start thread is specified, 
reduction of this outside diameter will result in serious weakening of the 
screw. In cases such as this a multi-start thread allows the requirement of a 
coarse pitch to be reconciled with that of a reasonable outside diameter. 

5.5.2 Lead and pitch 

The lead of a thread is the distance moved for each complete revolution 
of the screwed member; the pitch is the distance between equivalent points 
on a thread form. For a single-start thread the lead is therefore equal to the 
pitch. Fig. 5. 15A illustrates the set-up for cutting the first start ; note that the 
movement of the tool for one revolution of the work is 45 mm, and this is the 
lead of the thread. 



/£\ Cutting first start 45mm lea d 




&) Cutting second start -< 

Fig. 5.15.— Cutting First and Second Starts. 



5.5.3 Calculating the change gears 

The calculation of the change gears is a simple matter provided the correct 
formula is used : 

Drivers _ lead to be cut 
Driven — pitch of leadscrew 



114 WORKSHOP TECHNOLOGY I 

Assuming that the lathe leadscrew has a pitch of 5 mm, then 

Drivers 
Driven 



_ 45 
5 


= 9 
1 


= £x 
1 


3 
1 


- §2 
20 


x92 
30 



A compound train of gears can be made up, having 60- and 90- tooth drivers 
with 20- and 30- tooth driven gears. 

5.5.4 Calculating the depth of thread 

Reference back to fig. 5. 15B shows the fly-press leadscrew with the second 
start cut. Note that the pitch is equivalent to one-third of the lead, for 

lead of thread 



Pitch = 



number of starts 



The pitch in this instance is therefore one-third of 45 mm, which is 15 mm. 
The depth of thread of the three-start fly-press leadscrew can now be calcu- 
lated from the formula: 



Depth of thread = - 1-0-25 



mm 



! 5 1 
= — +0-2^ mm 

2 J 

= 7*5 + o-25 mm 
= 775 mm 

If we maintain the same proportions as we gave earlier when considering 
the making of a single-start 45 mm pitch leadscrew, namely having the core 
diameter equal to three times the depth of thread, the following will be the 
calculations for the outside diameter. 

Let depth of thread = d 
Then core diamerer = 3d 

and outside diameter = core diameter + 2d 

= %d+2d 

= 5^ 

= 5 x 775 mm 

= 3 8 '75 mm 

The advantage of multiple or multi-start threads will now be clear. If we had 
made our fly-press leadscrew by cutting a single-start thread of 45 mm pitch 
we should have had to use a 1 20 mm diameter bar, whereas if we cut a three- 
start thread we can use a 40 mm diameter bar. 

5.5.5 Locating the starts 

It is essential that the centre-lathe turner starts each cut in its correct posi- 
tion; that is to say, the pitch of the thread must be precisely 15 mm. This 



THE CENTRE LATHE 



"5 



means that, having cut the first start, we must now move the tool a distance 
of 15 mm towards the headstock of the lathe. 

There are several ways of achieving this accurate linear movement, and 
perhaps the simplest method is to index the compound slide forward a 
distance of 15 mm. This is easily achieved with the aid of the indexing dial, 
and it is a simple matter to check this movement using a dial indicator secured 
to the toolpost or lathe bed. Care must be taken to ensure that all backlash 
is removed prior to cutting the first start. 
15mm Slip gauges 

® 

Compound slide 



L 



£y 



-Tool 




Dial indicator 
to read zero 
before and 



after removal of 1 5 mm slip gauges 



1st driver 60 teeth 



Marks on gear 




Fig. 5.16.— Methods of obtaining Precise Tool Movement. 
Another method is to mark one of the teeth of the first driving gear against 
its mating position with the first driven gear. An indelible pencil or chalk 
mark will suffice. At the completion of the first start, the first driver is removed 
and the lathe spindle carefully rotated until the driving gear can be replaced 
exactly one-third of a revolution from the first position. Clearly it is necessary 
that the number of teeth in the driving gear be a multiple of three ; for example , 
if the compound train earlier calculated is used, then the first driver will have 
60 teeth, allowing the gear to be marked at intervals of twenty teeth. The 
two techniques described are simply illustrated in fig. 5.16. 

5.5.6 Grinding and setting of the cutting tool 

Considerable care is also required from the turner when grinding and 
setting the cutting tool that is to produce the Acme thread. Fig. 5.17 illus- 
trates the correct relationship between the tool section and the lathe centre 
line when viewed in the vertical plane. It will be seen that the helix angle, 
shown as 6 in the diagram, needs to be calculated if the cutting tool is to be 
ground with the correct clearance angles ; note also that the top face of the 
tool requires grinding so that it is at 90 to the helix angle of the thread. Once 



u6 



WORKSHOP TECHNOLOGY I 



again a knowledge of trigonometry is required ; the relevant calculations are 
given below. 




Helix 
angle 



Forward side clearance angle 



Front view of tool 

Fig. 5.17. — Effect of Helix Angle on Tool Shape. 

5.5.7 Calculating the helix angle 

Fig. 5.18A shows a cylinder which represents the lead of the thread at the 
core diameter. At B we see a development of this cylinder, from which it may 
be deduced that if the helix angle is equal to 0, then 



tan 6 = 



lead 
nd 



where d = core diameter of thread. 

If a clearance angle of 8° is required for both the forward and trailing sides 
of the thread cutting tool, then the following formulae may be used. 

Forward clearance angle = [ tan -1 — -7- j + 8° 



Trailing clearance angle = 



K^) 



-8° 



(Note: tan -1 is an expression or symbol for "the angle whose tangent is".) 
The clearance angles are indicated in fig. 5. 1 7B, where the forward clear- 
ance angle is shown as (), while the trailing clearance angle is shown as P. 
Remember that the view shown is a front elevation of the cutting tool ; when 
it is set in the lathe the operator or lathe turner has a reverse view of the tool. 
Although the example of a multiple or multi-start thread given in the 
preceding text may be considered as fairly extreme, producing a relatively 
fast or coarse-pitch thread, it should be clear that the cutting on a centre 



THE CENTRE LATHE 



117 



lathe of square or Acme threads, whether single- or multi-start, requires 
considerable care with respect to the calculation and subsequent grinding 
of the cutting-tool angles. 




Fig. 5.18. — Calculation of Helix Angle. 



5.6 Vertical boring machines 

Although the centre lathe may be considered as the most versatile of all 
the machine tools, it suffers from the disadvantage that large or heavy work 
requiring the use of the faceplate as a work-holding device poses a difficult 
problem to the centre-lathe turner, when one considers the difficulty of lift- 
int a heavy casting and clamping it to a vertical reference plane. Added to 
this there is the undesirable feature of deflection caused to the lathe spindle 
owing to the unsupported weight of the casting. These factors have led to the 




Fig. 5.19.— Typical Component suitable for Vertical Boring. 



n8 



WORKSHOP TECHNOLOGY I 



introduction of vertical boring machines, or vertical boring mills as they 
are more commonly called. 

In principle a vertical boring mill may be considered as a vertical centre 
lathe, with a large faceplate mounted in a horizontal position. With the face- 
plate in this position the loading and clamping of heavy work is greatly 
simplified, leading to safer working conditions for the operator, together with 



Column 



Boring F ocinq 
15-51 ^-*=""~ a 




Fig. 5.20.— Vertical Boring Techniques. 



THE CENTRE LATHE Iig 

much reduction in the time required to set up the work. The weight of the 
work to be machined is now taken directly by the bearing surfaces of the 
faceplate or worktable, with no forces causing spindle deflection. Thus a 
vertical boring mill is well suited for the machining of both internal and 
external cylindrical surfaces on large components. 

Such a component is illustrated in fig. 5.19; it will be seen that a tool 
movement vertically in the direction of arrow A is required for the machining 
of the external cylindrical surfaces, together with the internal cylindrical 
surfaces or bores. The plane surfaces will require movement of the cutting 
tool in a horizontal plane, as shown by arrow B. 

5.6.1 Types of boring mill 

The standard vertical boring mill is illustrated in simple form in fig. 5.20A. 
This type of machine is much used for medium to large work ; it has two 
tooling stations, allowing the machining of bores and external turning to take 
place simultaneously. Each tool post is carried on a rigid ram capable of 
vertical traverse under either manual or automatic feed. Both rams have 
independent movement along the cross-slide, with a range of automatic feeds. 
Slides are provided on the vertical columns, allowing the raising or lowering 
of the cross-slide to accommodate work of different sizes. The capacities of 
these machines vary ; the larger sizes represent some of the biggest machine 
tools in use, having worktables up to 8 metres diameter, and a working height 
of about 5 metres. 

5.6.2 Duplex boring mill 

A serious limitation of the boring mill just described is that the rotation of 
the worktable cannot be at the optimum revolutions per minute to suit two 
differing machining operations. If we refer to fig. 5.20A, it is evident that a 
considerable variation exists between the diameter of the work undergoing 
boring and that undergoing facing. If the cutting speed is correct (say) for 
boring, then this same speed will be much too slow for facing, and the 
efficiency of machining will be reduced. 

The duplex boring mill avoids this defect by employing two worktables, 
together with two tooling stations. It is important to appreciate that each 
ram is provided with separate driving and feeding arrangements, allowing 
the correct feed to be applied to suit the actual machining condition. The 
principle underlying the duplex boring mill is shown in fig. 5.20B. 

5.6.3 Vertical turret lathe 

Although called a vertical turret lathe, this machine tool is essentially a 
close relative of the boring mill. The advantages possessed by these machines 
consist in the provision of a multi-tooling device, more commonly known as 
a turret. This principle of multi-tooling is a necessary condition if efficient 
machining is to be achieved. The time taken to remove and replace a cutting 
tool can be considered as lost or unproductive, and perhaps the reader may 
have observed that all the machine tools so far described have been equipped 
with simple tool posts. While a 4-way tool post permits the presentation of 



120 WORKSHOP TECHNOLOGY I 

four tools to the workpiece, it is very seldom that the tools are set in position 
in these tool posts. 

The great advantage possessed by a turret tooling device is that the tools 
can be set in position and then indexed to the work in sequence. This allows 
the rapid and efficient machining of medium-sized components that require 
several machining operations, and we shall see, in the next chapter on turret 
and capstan lathes, the very considerable advantages offered by the correct 
application of turret tooling. 

Fig. 5.20C shows a typical turret as fitted to the duplex boring and turning 
mill illustrated in fig. 5.20B. Note that more than one tool may be accommo- 
dated in the tool-box ; although the turret has five tooling stations, more than 
five tools can be presented to the work. Each tool is presented to the workpiece 
by rotation of the turret, which indexes in a precise manner. This principle 
is ideal for the batch production of small to medium-sized components that 
require several operations on both external and internal faces. 

Summary 

If a small number of cylindrical formed workpieces are to be machined to 
close limits, the centre lathe cannot be equalled for this type of work. The 
advent of the axial flow jet engine has tended to re-assert the importance of 
the lathe as a machine tool, for engines of this type are required in relatively 
small numbers, necessitating the manufacture of the component parts in 
small batches. Although it is true that the lathes used for this purpose are of the 
turret or capstan type, the general principle of operation remains the same ; 
namely the presentation of a cutting tool to the rotating work. 

The great disadvantage of the centre lathe lies in the small number of tool- 
ing stations, and much of the time of the centre-lathe turner is taken up in 
removal, replacement and tool setting. We must also remember that linear 
control of the cutting tool is the responsibility of the turner; only with skilled 
and careful use of the indexing dials can diameters be held to within, say, plus 
and minus 002 millimetres. Yet we have seen that provided the turner is 
skilled and competent, he is able, with the aid of toolmakers' buttons and slip 
gauges, to bore holes to accuracies almost comparable with those obtained 
when using a jig-boring machine. 

He can also screw-cut single or multiple threads, and may choose either 
chucks, collets or centres as methods of work holding. It cannot be emphasised 
too strongly that the efficient use of a centre lathe demands a craftsman of the 
highest order. It is unfortunate that about 40 to 60% of his time is spent in 
unproductive work — in tool changing and setting, and in the skimming of 
diameters to establish datums. This has led to the use of capstan and turret 
lathes, and we shall deal with these in the next chapter. Finally the machining 
of large cylindrical work requires the use of vertical boring mills. The weight 
of the work acts axially along the spindle, and there is no tendency for spindle 
deflection. At the same time the loading and clamping of the work are greatly 
simplified. Several types of boring mills are in use, and we have seen for the 
first time the introduction of the turret as a device enabling the adoption of 
the multi-tooling principle. 



THE CENTRE LATHE 121 

EXERCISE 5 

i With a typical example, illustrate an application of each of the following workholding 
devices with respect to centre-lathe work : 
(i) faceplate, 
(ii) collet-type chuck, 
(iii) two-jaw chuck. 
2 Describe in some detail a sequence of operations for the machining of the component 
shown in fig. 5.21 at a centre lathe. 



ALL DIMENSIONS IN MM 



2 holes 
50±°"«*| 




GREY CAST IRON 



Fig. 5.21. 

3 Using a component of your own choice, illustrate the technique known as button boring. 
State the degree of accuracy expected, and explain why this method of machining holes to 
accurate dimensions is seldom adopted. 

4 Why is it essential to ensure that a lathe faceplate set-up for button boring is accurately 
balanced? Using a vector diagram, calculate the distance from the lathe centre at which two 
40 kg masses at an angle of 120 must be placed, so that a set-up of 120 kg acting at 250 mm 
from the lathe centre is accurately balanced. 

5 With components of your own choice, illustrate the application of three different types 
of lathe mandrels. What precautions must be taken when using a friction mandrel? 

6 A square thread having two starts of 12 mm pitch and an outside diameter of 50 mm is 
to be machined on a centre lathe with a leadscrew of 5 mm pitch. 

Calculate the following : 

(i) depth of thread, 

(ii) lead of thread, 
(iii) core diameter, 
(iv) helix angle. 

7 Make a neat front elevation of the cutting tool required to cut the square thread described 
in question 6, and insert, after suitable calculation, the clearance angle for both leading and 
trailing sides of the tool. (Note: angle to be calculated from top face of tool.) 

8 Explain why centre lathes are seldom used for the machining of relatively heavy 
castings. 



122 WORKSHOP TECHNOLOGY I 

9 With a simple diagram, illustrate the principle of a vertical boring and turning mill. 
Sketch, giving approximate dimensions, a typical component that would be suitable for 
machining on a vertical boring mill. 

10 What are the disadvantages of a two-head, single-table vertical boring mill, equipped 
with standard-type tool posts? Explain how the principle of multiple or multi-tooling is 
achieved on a duplex vertical boring mill, and state also the advantages to be gained when 
machining fairly large numbers of medium-sized cylindrical formed components. 



O Turret and Capstan Lathes 

6.1 Introduction 

We have seen, in our short discussion on the centre lathe in the previous 
chapter, that considerable skill and knowledge are required by the centre- 
lathe turner if he is to obtain the best results from his machine tool. Yet the 
actual time taken in the application of this skill and knowledge, namely the 
setting up of the workpiece and the cutting tools, constitutes unproductive 
time. We must not forget that a machine tool is a power-driven apparatus 
designed to produce geometrical surfaces, with metal being removed in most 
cases. If it is to operate under ideal conditions, then as much as possible of its 
time must be spent in removal of metal. 

It is clear that this fact was appreciated as far back as 1855, at least in the 
United States of America, where fully developed turret lathes were used for 
the manufacture of guns. It is worthwhile to recall at this stage the remarks 
made in an earlier chapter with regard to the manufacture of muskets having 
completely interchangeable parts. Before attempting to establish and main- 
tain a system of dimensional control using measuring instruments or limit 
gauges, it is first necessary to have machine tools capable of producing the 
component parts to the required accuracy. This accuracy concerns not only 
the geometrical surfaces of components but also their linear and angular 
dimensions. Of equal importance is the fact that the machine tools must pro- 
duce the parts economically ; that is to say at high production rates and using 
unskilled or semi-skilled operators. Experience suggests that on the average 
about 50% of the time spent in producing a component at a centre lathe is un- 
productive; this is owing to the necessity for continual tool changing and 
setting. It is clear that if very large numbers of cylindrical shaped components 
are required, such as are to be found in the moving parts of pistols and guns, the 
centre lathe must be replaced as a production machine tool. 



6.2 Turret and capstan lathes 

The purpose of turret and capstan lathes is to produce, with the aid of 
relatively unskilled labour, large numbers of repetitive parts of cylindrical 
shape to within the dimensional limits required. This is achieved by equip- 
ping both turret and capstan lathes with additional tooling stations, thus per- 
mitting the setting up of enough tools to complete the whole of the machining 
operation. These additional tooling stations are provided by a device known 
as a turret. The mobility of this turret determines whether the lathe is de- 
scribed as a turret or a capstan lathe. Perhaps a quick look at the sort of com- 

123 



124 



WORKSHOP TECHNOLOGY I 



ponents likely to be produced will assist in differentiating between these two 
machine tools. 

6.2.x The turret lathe 

Fig. 6.1 shows a component to be used in the undercarriage of an aircraft. 
The first batch is of 200 components ; all dimensions are to within fairly close 
limits, and the surfaces are required to have a fine tool finish. Note the overall 
length of this component. The turret lathe is designed especially for relatively 
large work, and this is achieved by allowing a hexagonal turret to be indexed 

A 

At 




Plan 
^ a V' ew 
4-way toolpost* 
or Front square turret' 
4 tool stations 

Fig. 6.1. — Tooling Stations on a Turret Lathe. 

to the revolving workpiece. This turret is mounted on a carriage which is 
able to traverse the whole length of the lathe bed ; this movement is indicated 
on fig. 6. 1 by the arrow A. A turret lathe is also equipped with a front tool post, 
or square turret as it is sometimes called, together with a rear tool post. Both 
front and rear tool posts are mounted on a cross slide, and have movement at 
90 to the lathe centre line. If the slide carrying the front tool post is equipped 
with automatic feed the machine is known as a combination turret lathe. 
Note that the turret lathe has eleven tooling stations. There are six positions 
on the hexagonal turret, four positions on the front tool post, and one position 
on the rear tool post. 

It is possible for a skilled turret-lathe turner not only to set the cutting tools 
in position accurately, but also to set and adjust stops so that control can be 
maintained over linear dimensions when several identical components are 
required. 

A simple outline drawing of a turret lathe is shown in fig. 6.2; note the 
great similarity to a centre lathe. As already stated, the main difference 



TURRET AND CAPSTAN LATHES 125 

consists in the replacement of the loose headstock or tailstock by a hexagonal 
turret. Mounted, as can be seen, on its own carriage or saddle, this turret is 
capable of movement along the bed of the lathe under either manual or auto- 
matic feed. At the end of one machining operation the turret can be rotated or 
indexed in a positive manner, and a fresh tooling device presented to the work. 
Provided the movements of the cutting tool have been controlled with the 
accurate setting of adjustable stops or indicating dials, the greater part of 
the working time is now spent on the removal of metal. 



Headstock 

(2) Turret 

(3) Square turret 
(front tool post) 




® 
(5) 



Pilot bar 
Saddle 



Rear tool post behind , , 

* in " 



Fig. 6.2. — Elements of the Turret Lathe. 

6.2.2 The capstan lathe 

We have seen that the best use of a turret lathe is for the machining of 
fairly large components. There are many instances, however, when very large 
numbers of small to medium-size components are to be machined from 
standard bar or rod. This work is best done by a capstan lathe. Reference to 
the component shown in fig. 6. 3 A will indicate clearly the different technique 
adopted in the design and utilisation of the capstan lathe. 

The component shown is a phosphor-bronze wormwheel casting, pre- 
viously illustrated in fig. 4. 1 when the use of high-efficiency cutting tools was 
being discussed. The floor-to-floor time for this component when machined 
on a capstan lathe is approximately 1 -6 minutes, with tolerances of plus and 
minus 001 millimetres maintained on the bore. It can be seen that relatively 
short movements of the cutting tool are required for this component ; the 
overall length of the casting does not exceed 45 mm. This is further shown in 
outline in fig. 6.3B, which also includes a brief description of the operations 
required and the tool positions. Note that the front tool post or square turret 
is not used. The facing of the faces C and B is achieved with the use of the 
reverse tool post, while all the other turning and boring is carried out from the 
hexagonal turret. 



126 



WORKSHOP TECHNOLOGY I 




COMPONENT 
115mm 



OPERATION 




O 



Chuck on diameter Q 



Rough faces B & C 



Rough turn D E G F 



Finish turn D E G F 



Bore D 



Remove 



Rear tool post 



Hex. Turret I 



Hex. Turret 2 



Hex. Turret 3 



HEXAGONAL 
TURRET 



FLOOR 

TO FLOOR 

TIME 

1-6 MINUTES 



Fig. 6.3. — Machining Technique on a Capstan Lathe. 

For small lengths of traverse such as are present on the phosphor-bronze 
casting, only relatively short movements are required from the tools situated 
in the hexagonal turret. 

Fig. 6.4 shows the arrangement for the location of the turret on a capstan 
lathe. Note that the turret is mounted on a slide which is clamped to the bed 
of the capstan lathe. This restricts the movement of the turret along its slide 



Turret rotates 



Turret slide 




Clamped to bed 

Fig. 6.4. —Details of the Capstan Lathe Turret. 

to several centimetres as long as the slide is clamped in position. While the 
turret lathe has a capacity limited only by the length of the bed, all capstan 
lathes have a fixed capacity with regard to the maximum and minimum 
movements possible from the tooling arrangements in the hexagonal turret. 
It must be remembered that capstan lathes, like centre lathes, are available 
in a range of sizes or capacities. 



TURRET AND CAPSTAN LATHES 127 

6.3 Control of dimensions 

All turret and capstan lathes are fitted with devices that permit fairly close 
control over the linear dimensions of length or diameter, and the type of device 
fitted varies a great deal from machine to machine. In general, however, it 
may be said that the following principles form the foundation on which most 
of the dimensional control devices are based. 

6.3.1 The use of stops 

Stops are widely used to obtain repetitive dimensions on similar com- 
ponents produced on capstan lathes by relatively unskilled operators. A 
popular type of adjustable stop is illustrated in fig. 6.5. The movement of 
the hexagonal turret alongits slide is controlled by adjustment of the threaded 
screw, which is locked in position by the locking nut. For each turret position 

Traverse 
for turret 
position 4 




Fixed — 
stop 



Adjustable- 
screw i 



Circular stop 
plate for six 
adjustable 
stops 



^Locking nut 
Capstan Slide 



-^U-Traverse of 

for turret position I 

Fig. 6.5. — Turret Stops. 



there is a stop available ; each stop is presented in the correct position relative 
to the fixed stop by rotation of a circular plate, actuated by the rotation or 
indexing of the hexagonal turret. 

It is the capstan setter who adjusts these stops when first setting-up the 
machine ; when this setting is completed a capstan operator takes over. Thus 

Traverse 




Fixed stop J 



Dial indicator 

Fig. 6.6. — Micrometer Stop. 



128 WORKSHOP TECHNOLOGY I 

this operator is continually engaged at the machine ; no time is spent in setting, 
tool changing and measuring of diameters for index setting. 

It is customary to provide the capstan operator with limit gauges of the 
type described in Chapter 3 ; this technique affords a relatively cheap and 
simple method of ensuring that dimensional control is being maintained. 
Dial-indicator-type stops may be used when high precision work is required 
on high-quality turret lathes ; the principle is illustrated in fig. 6.6. Instead of 
the operator relying on the abutment of the adjustable stop against the fixed 
stop to determine the final position of the transverse movement of the turret, a 
visual indication is obtained by traversing the turret until the pointer of the 
dial indicator reads zero. In this way variation of pressure against the fixed 
stop is avoided, with the result that more precise and consistent dimensional 
accuracy is obtained. 

6.4 Work holding 

The work-holding devices used on a capstan or turret lathe differ little 
from those used at a centre lathe. In order, however, to reduce the time re- 
quired and the effort to be expended by the capstan operator, most work- 
holding devices used on capstan and turret lathes are automatically opened 
and closed. Pneumatic power, namely compressed air at a pressure of about 
500 to 700 kN/m 2 , is used for this purpose, and the chuck or collet is opened 
and closed through the movement of a small lever. 

Fig. 6 . 7 illustrates the principles involved in the holding of work for capstan 
and turret lathes. 

6.4.1 Collets 

Always used for the holding of cold-rolled section or extruded section ; that 
is to say section of fairly close accuracy. Several types of collet are available, 
capable of accurate and rapid holding of work especially if the collet is power- 
operated. Some typical spring collets are shown in fig. 6.7A. 

6.4.2 Chucks 

Used for the gripping of larger-sized work, or more particularly for the 
holding of previously turned work. Both three- and four-jaw chucks are used 
on capstan and turret lathes. The three-jaw power-operated self-centring 
chuck is widely used on capstan lathes for the production of medium-sized 
components from bar stock. If the chuck is to grip a forging or casting for a 
first operation the jaws are hard and possibly serrated. Soft-jaw chucks are 
used to hold components which have already received a machining opera- 
tion ; this prevents damage to the surface of the work. The soft jaws are easily 
machined if a relatively awkward-shaped casting or forging requires to be 
gripped. Fig. 6.7B illustrates the use of chucks for capstan work. 

6.4.3 Fixtures 

A fixture is a device designed to locate and grip a workpiece. If it is found 
that the shape of a workpiece is such that difficulty is likely when attempting 
to hold it in a spring collet or chuck, it may be more expedient to hold the 



TURRET AND CAPSTAN LATHES 

Ground, cold roll ed, extrude d section ( A ) 

SPRING COLLETS 



129 




Push 



Pull 





tbiri 



Hard jaws Annular work 




Soft jaws 



4 




3rd angle 



Component in section 

Fig. 6.7.— Work Holding on the Capstan Lathe. 

component in a fixture especially designed for the purpose. Fig. 6.7C shows 
a typical fixture for use on a capstan lathe. 

6.5 Spindle speeds 

We have seen that both capstan and turret lathes are essentially improved 
versions of the centre lathe, the improvements consisting in the provision of 
increased tooling capacity together with control of tool movement by means 



I3O WORKSHOP TECHNOLOGY I 

of the adjustable stops. In this way a considerable amount of machining is 
possible, and may be carried out practically non-stop as soon as the work- 
piece is suitably chucked. However, as we have seen from earlier work, there 
is an ideal cutting speed at which metal should be removed, with the revolu- 
tions of the work proportional to the diameter turned. It is certain that 
cylindrical components which are to be produced on a capstan lathe not only 
will possess diameters of different sizes but also may require a range of 
machining operations, including turning, facing, drilling and reaming. 

Because the turret lathe is designed to accommodate mainly large-sized 
work, a wide range of spindle speeds is essential. A small-capacity capstan 
need not have as many spindle speeds. The diameter of work intended for the 
capstan lathe may not exceed 50 mm, while the diameters to be turned on a 
turret lathe may range from 50 to 800 mm. It is nevertheless a great advant- 
age if the spindle speed on a capstan lathe can be changed or reversed quickly 
and easily without stopping the machine and wasting production time. 

6.5.1 Speed-changing techniques 

Several types of speed-changing technique are in use. A popular method 
used in the medium-sized capstan lathe is the two-speed motor, driving a 
gearing arrangement providing six speed changes . Thus by altering the motor 
speed a total of twelve spindle speeds are made available; a typical range is 
from 1 to 30 rev/s. Table 6.1 shows how these speeds are arranged. Note 
that the twelve speeds are divided into three groups of four. Any particular 
group can be selected by the rotation of a lever at the front of the headstock, 
while movement of the two-speed motor switch together with the clutch 
lever gives the speed required within the group chosen. 





Motor 


Clutch 
lever 


Spindle speeds (rev/s J 


First 
group 


Second 
group 


Third 
group 


Bottom speed 
Second speed 
Third speed 
Top speed 


slow 
fast 
slow 
fast 


slow 
slow 
fast 
fast 


1 

1-4 

6 

9 


i-6 

2'5 

10-5 
16 


3 

4*4 
i8-5 
28 



Table 6.1 Arrangement of spindle speeds on a capstan lathe 

Selection of one of the groups shown provides adequate cutting speeds for 
turning, boring and reaming without the necessity of engaging speeds in 
either of the other groups. 

High-quality turret lathes are available with eleven spindle speeds, with- 
in a range of from 0-4 to 40 rev/s, which can be selected by the movement 
of a single lever. 

Further advances consist in providing the machine-tool operator with a 
preselective device. This means that the required speed can be selected by 



TURRET AND CAPSTAN LATHES 131 

rotation of a dial carrying the spindle speeds available. Clutch engagement 
and plunger movement together bring about the change of speed required, 
and there is no shock or slip at engagement. This speed changing may be 
carried out while a cut is taking place, and no discernible changeover point 
can be detected on the machined surface. 

It is important to remember that the prime purpose of modern speed- 
changing devices is to enable the maximum efficiency to be obtained from the 
machine tool. The provision of a spindle brake allows immediate stopping 
of the spindle ; speed changes are selected with rapidity and engaged with no 
stoppage of the machining processes ; it is a simple matter to throw the spindle 
speed into reverse, say for the removal of a tap during a tapping operation. 
All these innovations are calculated to increase the metal-removal ability 
of capstan and turret lathes, and under these conditions it is important that 
the tooling used be of the highest possible quality if full advantage is to be 
taken of the reduction of unproductive time. 

6.6 Standard tooling devices 

While standard-type single-point cutting tools may be used in the four-way 
tool post, or square turret as it is sometimes called, the cutting tools used in 
the hexagonal turret require special holding devices. Such equipment is 
considered as standard, and in addition to tool holders includes bar-stop and 
centre-drill holders, drill chucks, tap holders and extension arms. These and 
other equipment can be kept in stock ready to be drawn out and set up as and 
when required by the capstan setter. 

This principle of standardising the equipment to be used on a capstan lathe 
helps to reduce the overall cost of tooling, always an expensive item in the 
machining of repetitive products. In many cases the equipment is such that 
two or more tools may be applied to the workpiece at the same time; this 
principle is known as combination tooling. Before considering the types of 
tooling applied from the hexagonal turret, it may be advisable to separate 
capstan work into two main categories : 
(i) chuck work, 

(ii) bar work. 

6.7 Tooling for chuck work 

We have seen that both hard and soft jaws may be used, while the work 
held can be bar stock, previously turned work, or even castings and forgings. 
For this type or range of workpiece the knee turning toolholder is an essential 
piece of equipment. 

6.7.1 Knee turning toolholder 

Illustrated in fig. 6.8, the knee turning toolholder provides rigid support 
for a standard single-point cutting-tool bit. Additional support is given by the 
overhead pilot which locates in the bush shown in the diagram. Note the 
screw adjustment to the turning arm, permitting a fairly wide range of dia- 
meters to be machined. If required a boring bar may be set in the central 
bore, permitting a combination of turning and boring to be carried out 



132 WORKSHOP TECHNOLOGY I 

simultaneously. As already stated, this type of toolholder is suitable for most 
types of chuck work, and the rigidity is such that tungsten-carbide tools can 
be used to the fullest extent. 

The principle of tool-work application is similar to that of the centre lathe ; 
the tool is applied radially to the work. It is evident that if a heavy cut is taken 
on bar stock there will be a strong tendency for the bar to deflect or bend 
away from the tool. We will see, when we come to the toolholding for bar 
work, the technique adopted to prevent bar deflection under the influence 
of the cutting forces. 



Pilot bush 




Central bore 



Fig. 6.8. — Knee Turning Toolholder. 

6.7.2 Boring bars 

A typical boring bar, suitable for clamping in the knee turning toolholder 
previously described, is shown in fig. 6.9. Very fine boring is possible on the 
smaller-diameter holes, using a boring bar with a tungsten-carbide-tipped 
microbore unit capable of accurate adjustment. Normal boring for larger- 
diameter holes is achieved using boring bars similar in design to those adopted 
for centre-lathe work. 




-Adjusting screw 

Fig. 6.9. — Boring Bar. 
6.7.3 Extension arms 

Bolted to the hexagonal turret and accurately located by means of a register 
or spigot, extension arms are widely used for holding boring bars, centres and 



TURRET AND CAPSTAN LATHES 



*33 



other shank-type tools or toolholders. An extension arm is illustrated in 
fig. 6.10. 

6.7.4 Starting drill and holder 

All hole drilling at the capstan lathe should be preceded by a true start, and 
the device shown in fig. 6.1 1 is used for this purpose. Note the spade- type or 
flat cutter; this is usually ground at an angle of 120 





Fig. 6.10.— Extension Arm. 



Fig. 6.1 1.— Starting Drill and Holder. 



6.7.5 Additional equipment 

In addition to the few examples of tooling devices given above, all the usual 
centre-lathe tooling is used. This includes standard drills and reamers, 
taper sleeves or sockets, taps and dies. When a high rate of production is 
required both self-opening die heads and collapsible taps may be used. 

6.8 Tooling for bar work 

The use of standard -sized bars, cold rolled or extruded, is an essential 
feature of the work carried out on capstan lathes. In general the diameters 
are relatively small, and this means that spindle speeds will be fairly high. At 
the same time, if the bar diameter is on the small side it is certain to be deflected 
if heavy cuts are taken. Yet unless substantial cuts are taken, much of the 
efficiency of our elaborately tooled capstan lathe is lost. The use of a roller- 
steady turning tool-holder allows heavy cuts to be taken without deflection 
of the bar. 

6.8.1 Roller-steady turning toolholder 

The principle of the roller-steady turning toolholder is shown in fig. 6.12. 
Note that the tangential turning force, shown as Fin the diagram, is opposed 
by the forces exerted by the two rollers, indicated as A and B. Clearly the 
rollers and tool must be an integral assembly, or contained within a box-like 
structure. Both rollers are independently mounted on slides, adjustable 



134 



WORKSHOP TECHNOLOGY I 



D epth of cu t 




Roller 
A 



Tangentia 



Fig. 6.12. — Roller-steady Turning Toolholder. 

over a fairly wide range of diameters. If the toolholder is required to machine 
concentric to a previously machined diameter, the rollers are set to lead the 
cutting tool and bear on the machined diameter. This is shown in fig. 6. 1 3 A. 

Roller 




Burnished 
finish 



Fig. 6.13. — Rollers Leading and Following Tool. 

When heavy cuts are to be taken on bars held in a power-operated collet 
chuck, it is necessary that the cutting tool now leads the rollers as shown in 
fig. 6.13B. With the highly polished rollers following the cutting tool as 



TURRET AND CAPSTAN LATHES I35 

shown in the diagram, there is a tendency for them to burnish the machined 
surface, resulting in considerable improvement in the finish of the work- 
piece. This burnishing action is brought about by the pressure exerted on 
the surface of the work by the rotating rollers. 

We see now two advantages of the roller-steady turning toolholder when 
the rollers follow the cutting tool : 

(i) no deflection of the workpiece leading to inaccurate work, 

(ii) improved surface finish on workpiece. 

6.8.2 Roller-steady ending toolholder 

We have seen that an improved finish results when the rollers .follow the 
cutting tool. Unfortunately, however, when starting a heavy cut, the cutting 
tool makes contact with the work before the rollers are able to provide the 
required support. This means that the bar will deflect away from the cutting 
tool, only to be forced back as the rollers make contact. This sets up a con- 
dition which is known as ribbing since it results in a series of ribs on the 
diameter of the machined bar; this effect tends to diminish as the cutting 
tool continues its traverse. The use of a roller-steady ending toolholder to 
break down the bar end will prevent the unwanted condition described 
above, and fig. 6. 14 shows a typical device of this kind. Note also the methods 
of breaking down the bar ends prior to the application of a roller-steady 
turning toolholder. In each case the breakdown is achieved at high speed 
with the roller-steady ending toolholder; the length of traverse, which is 
quite short, is controlled by the adjustable stop. 



© 




Rollers in advance 
of cutting tool 



Radial tool 



FRONT VIEW 



Minimum 
Adeflecti'on 




Methods of breaking down 

Fig. 6.14. — Roller-steady Ending Toolholder and Breaking-down Techniques. 



I36 WORKSHOP TECHNOLOGY I 

The purpose of this breaking-down operation is to reduce the depth of cut 
taken by the cutting tool in the roller-steady turning toolholder, as shown in 
fig. 6.14B. It will be seen that only a small amount of metal is encountered 
by the cutting tool before the rollers make contact, and there is little or no 
tendency for deflection of the bar. 

Before leaving the subject of toolholders, perhaps it should be noted that 
the cutting tool, as shown in the roller-steady turning toolholder (fig. 6.12), 
is presented tangentially to the work. This promotes greater rigidity, as the 
cutting force acts through the strongest section of the tool with no tendency 
to cause bending or deflection of the tool. Although a somewhat special tech- 
nique is required in the sharpening of the tungsten-carbide-tipped tool, 
excellent results are obtained ; but it is necessary to have special provision for 
rapid withdrawal of the cutting tool during the return stroke, to avoid chip- 
ping of the cutting tool and marking of the workpiece. The cutting tool used in 
the roller-steady ending toolholder shown in fig. 6.14 is presented radially 
to the workpiece following centre-lathe practice. 

6.9 Self-opening diehead 

The cutting of a screw thread at the centre lathe is a somewhat lengthy and 
complicated process, demanding considerable skill and confidence from 
the centre-lathe turner. If an external screw thread is to be cut during the 
machining of a component at a capstan lathe, it is certain that a self-opening 
diehead will be used. There is little doubt that these dieheads provide the 
ideal method of cutting external screw threads both accurately and quickly. 
Simple to use, easy to set, with removable dies that can be quickly set up in a 
grinding fixture and precision ground to the required angle, self-opening 
dieheads are to be found wherever components are produced on capstan 
lathes. 

The advantages offered by the use of self-opening dieheads include accur- 
ate threads produced by adjustment of the cutting dies, together with con- 
siderable reduction in the machining time ; reversal of the lathe spindle is 
not required ; the dies open and may be withdrawn from the work by travers- 
ing the hexagonal turret back along its slide. 

6.9.1 Principle of self-opening dieheads 

The principle underlying the design of self-opening dieheads consists in 
the presentation of four chasers or cutting dies to the revolving work. These 
cutting dies may be presented radially or tangentially, as shown in fig. 6.15. 
Rotation of an adjusting screw permits movement of the dies either towards 
or away from the centre, and an indicating line normally reads at zero on a 
graduated scale when the dies are set at the correct diameter for the thread 
to be cut. 

In operation the diehead, mounted in a suitable holder on the hexagonal 
turret, is taken to the work, with the spindle speed reduced according to the 
diameter of thread required. Force must not be used ; gentle pressure will 
allow the leading edge of the dies to bite, and as the cut takes place only 
sufficient pressure to counteract the drag of the turret slide should be applied. 



TURRET AND CAPSTAN LATHES 

RADIAL TANGENTIAL 



137 




-Cutting dies 

Fig. 6.15. — Radial and Tangential Cutting Dieheads. 

When the traverse of the turret is arrested by its previously set stop, a 
section of the diehead continues to screw on, and in moving forward clears a 
holding device. At this point powerful springs immediately force the dies 
outwards, allowing the capstan operator to take the turret back. The dies 
must be closed before the diehead is used for the next threadcutting, and this 
closure may be effected with a small handle at the side of the diehead ; altern- 
atively an automatic tripping device may be used. 

A typical die as used in the radial diehead is shown in fig. 6.16. It will be 
seen that the cutting face (shown as the line CD in the front elevation) is 
inclined to the centre line of the work to be threaded (indicated on the dia- 
gram as the line AB and A^ ) . This inclination of the cutting face ensures that 

Third angle projection 



Cutting face 




Fig. 6.16.— Die for Diehead. 



138 WORKSHOP TECHNOLOGY I 

the actual cutting or removal of metal is done by the edge of the throat and 
the first full tooth. The remaining teeth are above centre and serve to act as 
a nut, engaging in the thread cut by the preceding full tooth ; in this way 
pitch accuracy is ensured. 

The dies must be kept sharp, but in no circumstances must they be 
sharpened by hand. Simple fixtures are available which ensure that the 
correct angles are ground on the dies, and the greatest of care must be taken to 
ensure that the angles are correct and appropriate for the metal to be 
threaded. 

6.9.2 Roughing and finishing 

In accordance with the well-established principle of roughing and finish- 
ing in order to obtain both maximum metal removal and an accurate, well- 
finished machined surface, provision is made for two cuts to be taken : a rough- 
ing and a finishing cut. Fig. 6.17 shows a view of the popular Coventry die- 
head. It will be seen that in addition to the closing handle shown on the 

Closing handle 

Graduated scale 



Detent pin 
handle 




Back plate 



Adjusting screw 



External scroll 



Shank 



Fig. 6.17. — Self-opening Diehead. 

external scroll, there is a detent pin handle attached to the back plate. This 
detent pin handle is used for the purpose of taking two cuts without disturbing 
the adjusting screw. 

The first cut is taken with the detent pin handle in the roughing position ; 
the second (finishing) cut is taken with the detent pin handle moved over to 
the finishing position. It is recommended that the diehead be in the open 
position when the changeover is made. It is also important that the diehead 
be used only when the detent pin holder is in either the roughing or finishing 
position, unless the diehead is equipped with a three-position system for the 
detent pin handle. 

6.10 Cutting internal threads 

The standard solid tap may be used when cutting internal threads at the 
capstan lathe. A low spindle speed is required, which must be reversed in 
order to secure removal of the tap. If a small-diameter thread is needed there 
is much risk of broken taps due to the inherent brittleness of small taps; and 



TURRET AND CAPSTAN LATHES 



*39 



for light tapping operations the tap holder shown in fig. 6. 18A may be used. 
The sleeve, as shown, is provided with a knurled finish allowing the operator 
to take a firm grasp when feeding the tap to the work. If it is felt that the torque 
is excessive, with the attendant risk of tap breakage, it is a simple matter to 
release the grip on the knurled sleeve. The sleeve now rotates freely, and 
reversal of the spindle allows withdrawal of the tap. 

6.10.x Tapping blind holes 

The device illustrated in fig. 6. 18B is very suitable for the tapping of blind 
holes. It is essentially a slipping-clutch-type tap holder, and can be set to 
slip at differing torques according to the size of the thread or the material 
being cut. Reference to the diagram shows that a strong spring presses against 
the slipping-clutch members. The pressure exerted by this spring can be 
varied by tightening or slackening the compression nut. Thus, once set by 
trial and error, the device can be used for the repetitive tapping of identical 
threads. In the event of the torque becoming excessive, the clutch member A 
rides over the clutch member B; reversal of the spindle together with reverse 
movement of the turret allows the tap to screw itself out. 

Hand qri 




Slipping-clutch Spring 



Fig. 6.18. — Slipping-clutch Type Tap Holder. 

6.10.2 Collapsible taps 

Provided the diameter of a threaded hole is in excess of about 25 mm, a 
collapsible tap may be used. The principle may be considered as the reverse 
process to that of the self-opening diehead ; namely the closing or collapsing 
of threaded dies actuated by the stopping of the turret or external pressure 
on the collapsible-tap device. A handle is used to move the dies outwards, 
where they lock at the pre-set diameter. 

6. 1 1 Examples of bar work on capstan lathes 

Perhaps the widest use for the medium-sized capstan lathe lies in the 



140 WORKSHOP TECHNOLOGY I 

production of cylindrical components from bar stock. As we have seen, 
collet, three-jaw, four-jaw, and soft-jaw chucks may be used to locate and 
grip the bar from which the components are to be machined. Before deciding 
on the actual tooling devices to be adopted, it is important to ensure that the 
capacity of the capstan lathe is such that all the tooling devices have freedom 
of movement and do not foul any part of the machine. Capacity charts are 
readily available from the makers of capstan lathes, giving full details of 
dimensions of the machine tool, together with maximum and minimum 
movements of turret, front tool post and rear tool post. 

Example 1 

Manufacture of the 3% nickel case-hardening steel bush blank illustrated 
in fig. 6.19. Large numbers of these bushes are required, and will be finish 
ground after case-hardening. It must be appreciated that the tooling arrange- 
ments shown in the table are chosen to show the versatility of the capstan lathe 
as a machine tool capable of efficient and economical machining. The method 
shown is neither the best nor the quickest way of producing the steel bush, 
but will serve to give an introduction to the simple techniques underlying 
capstan tooling and production. 

Hexagonal turret station 1 

Adjustable stop, set to length of component plus width of parting-off tool 
plus about 0-2 mm for facing. The exact amount is a matter of setting skill, 
and this is where the experience and initiative of the capstan setter avoid 
considerable wastage of metal. 
Hexagonal turret station 2 

Combined centre and facing tool. Traverse of turret controlled by adjust- 
ment and setting of the stop for turret 2. 

Hexagonal turret station 3 

Knee turning toolholder with drill in central bore. Diameter A machined 
together with rough drilling of hole. Stop set so that drill just clears overall 
length of bush. 

Hexagonal turret station 4 

Knee turning toolholder with drill in central bore. Diameter B machined ; 
drill opens out previously drilled hole. Stop set for linear dimension C. 

Hexagonal turret station 5 

Reamer to machine bore to size. 

Square turret 1 (front) 

Single-point tool set to machine chamfer D. Correct angle provided by 
the tool; length of chamfer obtained by adjustment of stop. 

Square turret 2 (front) 

Single-point tool to machine undercut E. Width controlled by tool, depth 
and position by stops. 



TURRET AND CAPSTAN LATHES 



141 




" 



K 



T 



Undercut E Chamfer D 



HEXAGONAL TURRET 


SQUARE 


REAR 


1 


Stop 


Chamfer 


Part-off 


2 


Centre face 


undercut 




3 


Turn dia A..drill hole 
(rouqh) 






4 


Turn dia B, drill hole 
(finish) 






5 


Ream hole 






6 









REAR 




r 



£. 



<Q 



I 4 




FRONT HEXAGONAL 

Fig. 6.19. — Set-up for Machining Bush on a Capstan Lathe. 
Rear turret 

Parting-ofF tool to complete machining of component. 

It must be stressed at this point that the preceding example shows only the 
application of some of the standard tooling devices illustrated in the pre- 
ceding pages. Many more tooling devices are available, including hollow 
mills, multi-toolholders, recessing tools and knurling tools. The list is much 



1 4 2 



WORKSHOP TECHNOLOGY I 



too great to consider at this stage in our studies, but excellent catalogues and 
descriptive material are readily available from the manufacturers of capstan 
and turret lathes. 




Set stop Hex. i 



Face and centre 
Hex. 2 



Turn outside diameters 
2 using multi-tool holder 

and drill hole 
Hex. 3 



Cut thread with sdf- 
opening die head 
Hex 4 

Ream hole Hex. 5 



IIV^S^SZSJ Chamfer Front turret I 






Commence part -off 
Rear turret 




Chamfer 

T 55 ^ Front 
£ turret 2 




Finish part-off 



FINISHED COMPONENT 



Fig. 6.20. — Sequence of Operations on a Capstan Lathe. 

Example a 

A brass union is illustrated in fig. 6.20. This component is machined from 
hexagonal extruded brass rod 40 mm across the flats. A hexagonal spring- 
collet chuck will be used to grip and locate the brass bar; the sequence of 



TURRET AND CAPSTAN LATHES 



143 



operations is shown in fig. 6.20. Note also the stages in the machining of this 
component at the capstan lathe. 

6.12 Machining times 

The time required to machine components on capstan lathes is usually 
described as the floor-to-floor time ; in other words it is the total time occu- 
pied in taking the forging or casting from the floor, machining it, and replac- 
ing the completed job back on the floor. (Suitable boxes or containers may, 
or course, be used instead of actually stacking finished jobs on the shop floor.) 

It is not difficult to calculate the floor-to-floor time for the machining of 
components on capstan and turret lathes, but it is one thing to arrive at a 
theoretical time and another thing to machine the job to within the limits 
laid down in the drawing. Considerable practical experience is the best 
teacher of the technique underlying the assessment of machining times 
that compare favourably with the actual times required to machine the 
components. 

The following notes will provide a simple guide to the usual technique 
adopted. 

6. 1 2. 1 Operating or non-machining time 

Non-machining time can be considered as the time required to present 
the tools to the work, and to chuck and unload the component. The following 
table gives an approximate idea of the time required to carry out some of the 
essential functions when operating a capstan lathe. 



Operation 


Time 


Bring bar up to stop 


10 seconds 


Index hexagonal turret 


5 seconds 


Index square turret 


5 seconds 


Change speed 


4 seconds 


Change feed 


4 seconds 



The above times vary according to the size both of the components to be 
machined and of the capstan or turret lathe, and it is only practical experi- 
ence that provides accurate and reliable figures. Provided the sequence of 
operations has been decided, however, it is a relatively simple matter to add 
all the non-machining times and thus arrive at a total non-machining time. 

6.12.2 Actual machining time 

This is the actual time that the tools are removing metal. The principle 
involved in the calculation of machining times is more commonly referred 
to as feeds and speeds. Let fig. 6.21 represent a reduction of diameter on a 



i 4 4 



WORKSHOP TECHNOLOGY I 



component machined in a capstan lathe. The work revolves at 8 rev/s, while 
the tool has a feed of 02 mm/rev. The distance shown as L in the diagram 
is the traverse of the tool. It is required to calculate the time taken by this cut. 

I OO mm 




8 rev/s 



Number of rev/s = L inm m 

Feed per second 

Time taken ss Total number of r evs 
rev/s 

Fig. 6.21.— Calculating Machining Times. 

For each revolution of the work the tool advances a distance of 02 mm. 
Thus the number of revolutions of the work in a tool movement of L mm is 
equivalent to the distance L divided by the feed of 0-2 mm, giving the formula : 

Number of revolutions = T — t 

teed per rev 

_ 100 

~~ 0-2 

= 500 

We see from the diagram that the work revolves at 8 rev/s. Thus the time 

taken for the tool to traverse the distance of 1 00 mm will be slightly in excess 

of one minute; using the formula, 

„. , , • \ total number of revolutions 

lime taken (min) = =— - n 1 

v ' spindle rev/s 

_ 522 

~ 8 

= 62-5 seconds 

Adding together all the non-machining times and all the calculated 
machining times gives the total production or floor-to-floor time for the 
component. It must, however, be remembered that the hexagonal turret is 
equipped with automatic traverse, as is the cross feed for the front square 
turret. This means that the capstan operator can machine with the front 
turret while the hexagonal turret cuts under automatic feed. 



TURRET AND CAPSTAN LATHES I45 



Summary 

Turret and capstan lathes represent the basic machine tools designed for 
the efficient and economical machining of engineering components possess- 
ing mainly cylindrical shapes. The turret lathe has a hexagonal turret which 
is capable not only of being indexed in six machining positions, but also of 
complete movement along the whole length of the bed of the lathe. 

In general the capacity of turret lathes is large. They are eminently suitable 
for the repetitive machining of fairly large precision components, typical 
examples being provided by the component parts of aircraft, turbines, and 
elements of machine tools. In all the examples just given, the number of 
components required is not likely to exceed (say) several hundred, and it is 
very likely that the production of the parts will be divided into small batches, 
say 40 per batch. This technique will permit the improvement or modifica- 
tion of the component in the light of experience during manufacture and in 
service. 

The use of capstan lathes differs from the use of turret lathes mainly with 
respect to the size of the component to be machined. The traverse of the 
hexagonal turret is relatively restricted, although the slide on which the 
turret moves may be clamped at any convenient place on the lathe bed. 

The great advantage possessed by both turret and capstan lathes is the 
number of tooling stations available. In the case of a capstan lathe, instead of 
the operator spending unproductive time in tool setting and tool changing, 
this work is done by an experienced and skilled setter. When the lathe is 
completely tooled, with the stops set to give the required linear distances, the 
capstan operator takes over the machining of the component. There is thus 
little non-machining time, and the greater part of the time is spent in the 
removal of metal. A wide range of tooling devices can be presented to the work 
in sequence. Threads are easily cut, holes may be drilled, reamed or bored, 
and chamfers machined both internally and externally. The machining of 
a complicated profile is readily achieved with the use of a form tool, held in 
either the front or rear tool post or the square turret. 

It is important to appreciate that the capstan lathe requires a skilled setter, 
who is responsible for the setting of the tools and stops, while a capstan 
operator carries out the machining of the components. The procedure when 
producing components from the larger-type turret lathes is somewhat differ- 
ent. Here the turret-lathe operator is a highly skilled man. He both sets and 
operates the turret lathe ; the tooling devices and stops are used to reduce 
the time and cost involved in the machining of several fairly large identical 
components required to close dimensional accuracy. 

Finally, it is possible to estimate, with some degree of accuracy, the time 
required to produce a component on a capstan lathe. A list of the machining 
operations is drawn up and the total time calculated. This allows a measure 
of cost estimating to be carried out, and in this way a firm is able to judge 
whether it is an economic proposition to accept a sub-contract for a large 
number of components at a stated price. 



146 



WORKSHOP TECHNOLOGY I 



EXERCISE 6 

1 What is the essential difference between a turret lathe and a capstan lathe? Illustrate 
with diagrams. 

2 Explain the advantage possessed by a capstan lathe with respect to tool holding. 

3 Make a neat sketch of a component that would be suitable for production on : 

(i) a capstan lathe, 
(ii) a turret lathe. 

4 Explain how dimensional control is maintained when producing components on both 
capstan and turret lathes. 

5 Show, by means of sketches, typical applications of the following work-holding devices : 

(i)' spring-collet chuck, 

(ii) three-jaw soft-jaw chuck, 
(iii) three-jaw hard-jaw chuck, 
(iv) turning fixture. 

6 Make a neat sketch of the following tool-holding devices, showing a typical application : 

(i) knee turning toolholder, 
(ii) roller-steady turning toolholder. 

7 Show, by means of a neat layout, the tooling arrangements suitable for the production 
of the component shown in fig. 6.22 from hexagonal bright mild steel. 



48 



ALL DIMENSIONS IN MM 



38 



25 



MIO 




Make from 25 A/F H«x Mild steel 
All tolerances ± 0*12 

Fig. 6.22. 



8 Describe the conditions best suited for the application of a roller-steady turning tool- 
holder using a tangentially inclined tool. What precautions are necessary before : 

(i) taking a cut, 
(ii) returning the turret to the starting position? 

9 Make a neat sketch of a component to be produced on a capstan lathe that would 
require the use of a self-opening diehead and a collapsible tap. 

io Why is it necessary to be able to calculate the floor-to-floor times or components pro- 
duced on capstan and turret lathes? Outline the use of the study of feeds and speeds with respect 
to the calculation of machining times. 



I Hole Production 

7.1 Types of drilling machine 

Fig. 7.1 shows three typical engineering components possessing holes. The 
production of holes in engineering components such as these constitutes a 
most important aspect of engineering manufacture, and several techniques 
are adopted. We have already dealt with the use of sensitive, pillar, and radial 
drilling machines in Workshop Processes 1 and 2, and perhaps it will be re- 
membered that of the three drilling machines described, only the radial 
drilling machine adopts the principle of taking the tool to the work. 
Oil hole 



Ream for dowels. 




C/bore 
other side 



Fig. 7. 1. — Engineering Components Requiring Holes. 



A brief survey of standard drilling machines at this stage will serve to 
remind the student of their principal uses. 

7. 1. 1 Sensitive drilling machine 

Both bench and pillar types available. Three to four spindle speeds with 
relatively high revolutions; speed changes effected by belt changing on 
coned pulleys. Suitable for the drilling of small-diameter holes; vertical 
feed obtained by hand pressure. Not suitable for reaming, counterboring 
or the drilling of large-diameter holes. 



H7 



148 WORKSHOP TECHNOLOGY I 

7.1.2 Pillar drilling machine 

Available only as a pillar type. These drilling machines are much used for 
the average type of work, including drilling, reaming, counterboring, 
counter-sinking and the tapping of holes. Automatic feeding of the spindle 
is available together with a choice of feeds. Speed changes effected through 
gearing arrangements, but the spindle may still be fed by hand (though with 
loss of sensitivity). Unless the table is of the compound type, that is to say 
unless provision exists for movement at 90 , the drilling of holes to close 
linear dimensions is a difficult matter. 

7.1.3 Radial drilling machine 

Operating on the principle of taking the spindle to the work, the radial 
driller is much used for the drilling of holes in large castings. A wide range of 
spindle speeds, together with automatic feed of the spindle, enables the radial 
driller to deal with most of the drilling required in large castings or forgings, 
but once again the drilling of holes having their centres within close limits is 
a difficult task. We will see later on in this chapter the principle underlying 
the use of ajig borer for the production of holes not only with precise diameters 
but also having precise linear distances between centres. 

7.2 Piercing holes 

This technique is the quickest method of producing holes in relatively 
thin material. Reference back to fig. 7. 1 A shows a typical component having 
several pierced holes. The part shown is an outer ring for the hub of a bicycle 
wheel. If we count the large-diameter hole at the centre, there are 1 7 holes in 

Punches 



Land 

Stock component 




Component . , . 

Make from32x4mmBMS 

Fig. 7.2.— Technique of Piercing Holes. 



HOLE PRODUCTION I49 

this component ; yet the production time for these holes is not likely to exceed 
more than a few seconds — the time required for the stroke of a press. 

The principle of the piercing operation is shown in fig. 7.2A. The holes are 
produced with the combination of a punch and die ; for the wheel component 
shown, 17 punches are required with 17 holes in the die. Both punches and 
dies are part of an accurate press tool, the punches being integral with the 
top tool while the die is integral with the bottom tool. 

The top tool is mounted in the ram of a press and is accurately aligned 
by means of pillars so that the punches enter their respective holes in the dies 
with no errors of alignment. On the closing of the tool or the downward stroke 
of the ram, the metal stock from which the component is produced is subject 
to severe stress, resulting in failure of the metal across the shear plane. The 
area of this shear plane is equal to the perimeter Or circumference of the hole 
multiplied by the thickness of the metal. Thus if the small holes are 4 mm 
diameter and the large hole is 25 mm diameter, the total shear plane or area 
is equal to : 

7rZ)X4+ 167^x4 
47i(D+i6d) 

Substituting actual diameters : 

Shear area = 471(25-1-16x4) 

88 
= yx89 

= 1 1 18 mm 2 (approx.) 

With the total shear area known it is a relatively simple matter to calculate 
the theoretical force required to stamp out or pierce the 1 7 holes. 

Shear stress = 

area 

.'. force = shear stress x area 
Given that the mild steel from which our component is made has a shear 
stress of 400 N/mm 2 , 

Force to pierce holes = shear stress x area of shear 
= 400 x 1 1 18 newtons 
= 447 kN 

Note the small amount of land, followed by a gradual taper or enlargement 
of the holes in the dies. When the hole is pierced, the unwanted metal, or 
slug as it is sometimes called, is forced into the die. It is in a state of compres- 
sive stress, and thus presses tightly on the face of the hole in the die. If this 
hole in the die is parallel throughout its length it is certain that a build-up of 
slugs will take place within it, leading to blockage and subsequent cracking 
of the die. 

The small land, usually about i£ T, where 7" is the thickness of the metal 
to be sheared, ensures that the size of the hole will remain constant even 



150 WORKSHOP TECHNOLOGY I 

after repeated grindings of the top face of the die (a necessary process if the 
die is to be kept in a sharp condition) . The top tool is set so that the punch, at 
the end of the stroke, pushes the slug just clear of the land. At this position the 
slug begins to clear the hole owing to the angle of taper machined on the hole, 
and the slug is free to fall into a suitable receptacle placed beneath the press. 
This technique of producing holes is widely adopted in the manufacture of 
engineering components. Production rates are extremely high, and the 
process can be completely automatic. The dimensional accuracy of the hole 
sizes and positions remains constant even after many thousands of com- 
ponents have been produced, an important factor in the mass-production 
of components. The need for measurement of the component is eliminated, 
while inspection need only be carried out at intervals. The only limitations 
are the thickness of the metal from which the part is to be made, and the suit- 
ability of the design of the component for press-tool manufacture. 

7.3 Use of drill jigs 

Fig. 7.1B shows a part produced at a capstan lathe, and it will be seen that 
a further oil hole is required. Perhaps it has not been appreciated that the 
manufacture of components at a capstan lathe is made possible because the 
datum of the work is the centre line, which is common to all the turning 
operations. The geometric accuracy of all the external diameters, and that 
of bored or drilled holes, is controlled by their relationship to the centre line 
of the work. If, however, a hole is required with an axis that does not coincide 
with the centre line of the work, then a totally different situation exists. It is 
unlikely that such a component can be finish machined at the capstan lathe. 

I 04 ° - | [ ALL DIMENSIONS IN MM 




of work 



AH tolerances t 005 
Phosphor Bronze Bush 

Fig. 7.3. — Component Requiring Drill Jig. 



HOLE PRODUCTION I5I 

A further drilling operation is necessary. Fig. 7.3 shows the phosphor-bronze 
bush already illustrated in fig. 7.1B. While the 40 mm diameter bore can be 
bored from the hexagonal turret concentric with the external diameter of 
50 mm, it will be seen that the 5 mm diameter hole has its axis at 90 to the 
centre line of the bush. 

This fact, that each and every hole has its own geometric conditions or 
accuracy, makes the production or machining of holes a difficult task, and 
one that has concerned engineers for a good many years. Because the datum 
of a hole is its centre, it is a very difficult matter to produce holes having their 
centres to within close linear dimensions with regard to other hole centres or 
datum faces. Immediately the hole is machined the centre becomes non- 
existent or imaginary, and it is necessary to finish machine the hole before 
determining the accuracy. 

This is unlike the technique of turning, milling or grinding, where cuts can 
be taken, the resultant size checked with a measuring device, and the re- 
quired dimension obtained by suitable movement of cutting tool or work- 
table achieved by careful indexing. 

In the drilling or machining of holes it is essential that the machining of 
the hole be commenced in the exact position. Thus the drilling of the 5 mm 
diameter oil hole in the phosphor-bronze bush illustrated in fig. 7.3 involves 
the positioning of the drill not only on the centre line of the bush or dia- 
metrally, but also at a distance of 12 mm plus and minus 0-05 mm. When 
large numbers are required, an efficient, accurate and economical method 
of producing holes is to make use of a drill jig. 

7.3.1 Principle of drill jigs 

The purpose of a drill jig is to provide location and support for a compon- 
ent while one or more drills are guided in predetermined positions. This 
means that the drilling of holes to accurate dimensions is now achieved by 
employing a device which has been constructed to a very high degree of 
accuracy, the actual operation of drilling being reduced to a simple matter 
of feeding the drill through the metal. The accuracy is obtained by ensuring 
that the component is correctly and positively located, and also securely 
held or clamped while the drilling takes place. 

Fig. 7.4 shows a simple drill jig suitable for the drilling of the hole in the 
phosphor-bronze bush. Note that the hardened and tempered drill bush is a 
press fit in the top plate, with the centre of the drill bush at a distance of 12 
mm from the face of the locating pad. 

The centre line of the drill bush is also at 90 to the axis of the spigot on 
which the phosphor-bronze bush locates, as shown in fig. 7.4A. Looking then 
at view B we see that the centre line of the drill bush passes through the centre 
of the spigot and is at 90 to the base of the drill jig. As already stated, the 
whole purpose of the drill jig is to transfer these accurate alignments to the 
component being drilled, and provided the component is brought up to the 
location pad and held there in close contact by the tightening nut, consistent 
accuracy is achieved. 



152 WORKSHOP TECHNOLOGY I 

Note that the bore of the phosphor-bronze bush exceeds the diameter of 
the tightening nut, and that a C washer is used to hold the component hard 
against the location pad. At the finish of the drilling operation the tighten- 
ing nut is unscrewed about one turn or less, the C washer lifted off and the 
component removed from the spigot, passing over the tightening nut. In 
this way the drilling of the hole becomes a matter of routine, and provided 
the face of the locating pad and the ends of the component are both kept 
free from swarf, a high rate of production is possible, with the added advant- 
age that work produced by relatively unskilled personnel will be consistently 
within the limits. 




/J\ WELDED CONSTRUCTION ^g\ 



Fig. 7.4. — Typical Drill Jig. 

Although a wide variety of drilling jigs are in use, basically the principle 
remains the same: the transfer of accurate geometric conditions to the com- 
ponents within the jig. Provided the holes required are the same diameter, 
there is practically no limit to the number of drill bushes that may be incor- 
porated in a drilling jig, and if the jig is capable of being fairly easily handled 
by the operator a single-spindle drilling machine may be used. If, however, 
several holes of different diameter are required in a component, or differing 
operations are required on the holes, then a single-spindle drilling machine 
will be of no value. This rules out the use of sensitive, pillar and radial drillers ; 
we need now a multiple-spindle drilling machine. 

7.4 Multiple-spindle drilling techniques 

As the name suggests, these drilling machines are equipped with more 



HOLE PRODUCTION 153 

than one spindle. Two main types of multi-spindle drilling machines are in 
use: 

(i) in-line or gang drilling machines, 

(ii) multiple-head drilling machines. 
Both types are essentially production machines; that is to say their use is 
confined to the drilling of holes on a mass-production basis. 
7.4.1 The gang drilling technique 

Gang drilling involves the use of two or more drilling spindles which are 
in line and part of the same drilling machine. Thus, having drilled one hole, 
the operator slides the work to the next spindle and proceeds to drill 
or machine a further operation on the previously machined hole. The com- 
ponent illustrated in fig. 7 . 1 C is a good example of the need of a gang drilling 
technique. This component, a mild-steel forging, has been machined on a 




4 Holes drill 
0IOc'bore0l5 



2 Holes drill & 
ream IO 



ALL DIMENSIONS IN MM 



Fi g- 7-5— Component Requiring Drilled, Reamed and Counterbored Holes. 



capstan lathe, and it will be seen that four further holes require to be 
machined. Note that these holes require counterboring ; note also the two 
dowel holes, which will require reaming. 

Four separate operations are required on this component : 
(i) drill four 10 mm diameter holes, 
(ii) counterbore four holes of 15 mm diameter, 

(iii) drill two holes, 9-2 mm in diameter, 

(iv) ream two holes of 10 mm diameter. 
A drilling jig is required, and must be designed to locate and clamp the 
machined forging further illustrated in fig. 7.5. 

We are not at this stage concerned with the design and manufacture of 
the drilling jig required, although this kind of work would prove a useful 
exercise in the application of some of the principles and techniques covered 
in this and previous volumes. 



154 



WORKSHOP TECHNOLOGY I 



We see from fig. 7.5 that six holes require to be machined, as follows: 

(i) four holes drilled 1 o mm diameter and counterbored 1 5 mm diameter ; 

(ii) two holes drilled 9-2 mm, and reamed 10 mm diameter. 
All the above operations are carried out using the same drilling jig and in one 
clamping of the component. The essential stages are shown in fig. 7.6, where 
it can be seen that four spindles are used. The procedure is as follows : 
(i) drill four 10 mm diameter holes; 

(ii) drill two holes 9-2 mm in diameter, 

(hi) remove drilling bush for 10 mm diameter holes, replace with 15 mm 
counterboring bush and counterbore four holes, 

(iv) remove 9-2 mm drill bush and replace with 10 mm diameter reaming 
bush ; ream two holes. 

Note the use of slip and liner bushes, as shown in fig. 7.6B. The liner bush 
is a drive fit in the top plate of the drilling jig; a good example of the need for 
precision machining to produce a desired fit. The slip bush is a push fit into 
the liner bush, and once again* we see the need for precision work in the 
manufacture of engineering components. 

The gang drilling machine may be equipped with guide strips to help 
locate the drilling jig and prevent its rotation. Each spindle has independent 
drive, and the rev/s are set to suit the particular operation in hand. 

Once again the holes required in the forging shown in fig. 7.5 may be 
drilled, counterbored and reamed, not only to a consistent degree of accur- 
acy, but also at a high rate and using relatively unskilled personnel. If, how- 
ever, as is common practice, one operator carries out the drilling described, 
then a certain disadvantage results. Although the drilling machine used 

3 2*ftl 4 




Drill0IOhole Counterbore 15 Drill 09-2 Ream0IO 



Top plate 
of drilling 
J»9 



ALL DIMENSIONS IN MM 

Fig. 7.6. — Use of Slip and Liner Bushes. 




Non-rotate 
slip bush 



HOLE PRODUCTION 



*55 



has four spindles, only one spindle is in use at a given time. This means that 
three spindles are rotating but not actually employed in the removal of metal, 
so that the operation of the drilling machine is not as efficient as it might be. 
This defect has led to the introduction of multi-head drilling machines. 

7.4.2 Multiple-head drilling machines 

These drilling machines represent perhaps the most efficient method of 
hole drilling. Several drills are presented to the workpiece simultaneously ; 
there is no idle spindle time except during the loading and unloading of the 
component. 

In both the previous examples of drilling techniques, consistent accuracy 
is achieved with the principle of drill-guiding or the use of hardened and 
tempered drill bushes. Multi- or multiple-head drilling machines need no 
drill jigs or drill bushes. Holes of different diameters, counterbores and tapped 
holes are readily machined in the one setting. For very high production 
figures upwards of 100 holes may be drilled simultaneously using special- 
purpose machines. 

7.5 Broaching holes 

The technique of broaching represents a marked departure from the more 
orthodox method of finishing a hole by rotation of a cutting tool such as a drill 
or reamer. It is important to appreciate that broaching holes is essentially 
a finishing process— the hole must already be machined to close on finished 
size. The normal method of giving a drilled hole a good finish is to pass a 
suitable reamer through it at slow rev/s and a fairly slow feed. This is a slow 



Rake 




Finishing 
teeth i 



Semi-finishing 
teeth [ 



Roughing 
teeth 



Component 



Fig- 7.7.— Technique of Broaching Holes. 



156 WORKSHOP TECHNOLOGY I 

process, and any wear on the reamer must result in the production of under- 
size holes. Although machine reamers are provided with a taper, allowing 
the leading edge of the reamer to do most of the roughing, with less work for 
the latter part of the reamer, rapid wear takes place if the reamers are used 
for mass-production purposes. Thus the machining of holes to close dimen- 
sional limits and possessing a good surface finish is no easy matter if reamers 
are used as a method of finishing previously drilled holes. 

Broaching has largely replaced reaming when large numbers of holes are 
required, because a well-made broach has a long and accurate life ; the broach 
is equipped with both roughing and finishing teeth. A typical broach is 
illustrated in fig. 7.7. This broach is a simple pull- type, that is to say the 
broach end is fed through the hole, joined to the pulling device by a key and 
pulled through the hole. 

Note that the operation shown in fig. 7 . 7 consists in broaching a round hole, 
and the broach is circular. At B we see a closer view of the broach teeth ; note 
that succeeding teeth are of larger diameter, with each tooth performing a 
given amount of work or removing a fixed quantity of metal. The first third 
of the broach is made of roughing teeth, which remove more metal than the 
semi-finishing teeth which follow them. Lastly the finishing teeth complete 
the work, and these teeth remove a relatively small amount of metal, with the 
last three or four teeth on finished size. 

In this way the finishing teeth, which control the size of the hole and thus 
the life of the broach, have very little metal to remove ; the greater part of the 
metal removal is achieved with the roughing teeth, and this means that the 
broach has a long, accurate life, and is capable of producing a very large 
number of well-finished and close-dimensioned holes in the minimum of 
time. It must be remembered, however, that the manufacture of broaches 
represents a very high standard of precision engineering, with the result that 
broaches are costly pieces of equipment and must be handled and treated 
with the greatest of care. Broaches are also eminently suitable for the produc- 
tion of the internal splined form shown in fig. 7.8, the broach having a similar 
cross-section to the internal profile of the component. 

-Splined hole 
broached 




Fig. 7.8.— Component requiring a Splined Hole. 



HOLE PRODUCTION 157 

7.6 Jig boring 

All the drilling examples given above refer to the production of holes on a 
production basis. The aim at all times is to produce well-finished holes with- 
in the limits laid down. We have seen that this is achieved with the aid of drill- 
ing jigs or multiple-head drilling machines, in which the accuracy required 
is built into the jig or machine used. 

Clearly the manufacture of these devices requires the drilling of holes to 
very close dimensional limits, work which is quite outside the scope of 
sensitive, pillar and even radial drilling machines. When holes are required 
to very close dimensional tolerances with respect to both diameters and hole 
centres, the technique used is more usually that known as jig boring. 

7.6.1 Principle of jig boring 

Although several types of jig borer are available, there is a common prin- 
ciple underlying their design and operation ; namely precise linear control 
of the worktable together with vertical control of the spindle. The difference 
between the types of jig borer is determined by the methods adopted to 
obtain precise control over the linear movement of the worktable; these 
movements are shown in simple form in fig. 7.9. It is clear that a jig borer is 
somewhat similar to a compound-table drilling machine, except that it is 
of much more rigid construction, with more positive methods of controlling 
the worktable movement. 




F'g- 7-9— Co-ordinate Movements when Jig Boring. 

The location of the first hole is determined by the linear dimensions A and 
B, both taken from the ends of the component, which may be considered as 
datum faces. Note that the positions of the other two holes are controlled by 
the linear dimensions C and D. The location of these three holes is a good 
example of the principle of rectangular co-ordinates introduced in Work- 
shop Processes 2, Chapter 5, page 84. The purpose of a jig borer is to make 
possible the positive adjustment of the worktable in the directions of the 
arrows in fig. 7.9, to an accuracy of plus and minus two micrometres. 



158 WORKSHOP TECHNOLOGY I 

In general two systems are in use by which accurate linear control of the 
table can be maintained : 

(i) the use of end standards, 
(ii) the use of scale and microscope. 

7.6.2 End standards method 

Fig. 7.10 illustrates in a simple manner the principle underlying the use of 
end standards as a means of controlling the linear movement of the work- 
table in a jig boring machine. At A we see the set-up for the drilling of the first 
hole. Note the dial indicator securely clamped to the fixed member of the 
worktable, and the adjustable stop attached to the moving worktable. If 
the centre distance between the two holes is to be 127595, then a slip build- 
up of this amount is placed in position, and the worktable is brought up 
until the dial indicator reads zero. 

The first hole is now drilled to the required diameter, or a boring head may 
be used. The problem involved in moving the worktable a linear distance of 
127-595 i s now easily solved ; it is only necessary to remove the slip build-up 



127*595 I ALL DIMENSIONS IN MM 




Distance moved by 
table— 127-595 



Slip gauscs removed, table moved to zero reading 

Fig. 7.10. — Control of Linear Table Movement when Jig Boring using 
Slip Gauges and Dial Indicator. 



HOLE PRODUCTION 



159 



and bring up the worktable until the dial indicator reads zero on contact 
with the adjustable stop. The second hole may now be machined with 
complete assurance that the table has been moved the amount required. 

A similar method is adopted to ensure that the movement of the worktable 
is also controlled at 90 to the first movement, and if the centre distances of 
the holes are in excess of 250 mm, end measuring bars are employed ; their use 
has already been discussed in Chapter 2. Note that the dial indicator in the 
set-up shown in fig. 7.10 is used as a zero indicator and not as a measuring 
device. Its purpose is to eliminate the variation in pressure that is likely to 
take place when the operator brings the table up against the fixed stop ; the 
dial indicator now acts as a tell-tale. 

Another method of controlling the worktable movement of a jig borer in- 
volves the use of precision hardened rollers of 25 mm diameter. These rollers 
are accurate to within 05 urn, and are chromium-plated to increase their 
working life. 

Each division 0005 / MICRO-LOCATOR reading ,13-185 




ALL DIMENSIONS IN MM 



Worktable 



Fig. 7.1 1.— Application of Precision Rollers and Micrometer Head 
for Precise Table Movement. 

Fig. 7.1 1 shows the basic principle involved. Each roller provides an inch 
measuring unit, and location of the table to within less than 25 mm is obtained 
by means of the micro-locator shown in the diagram. This is a micrometer 
device with a large-diameter thimble which provides accurate readings to 
two micrometres. Once again dial indicators are used as zero positioning 
devices. With a micro-locator placed each side of the worktable, movement 
of this table is achieved by moving the micro-locator across the rollers giving 
25 mm steps, and then using the micrometer to obtain the remainder of the 
distance required. The diagram shows the set-up for a table movement of 
88- 1 85 mm ; note that the dial-indicator device is not shown. This technique 
is a simple but effective method of controlling the movement of the work- 
table, and the drilling and boring of holes having their centres to within a 
tolerance of plus and minus two and a half micrometres presents little 
difficulty. 

7.6.3 Scale and microscope method 

As the name suggests, this method makes use of a fixed scale or line standard 



i6o 



WORKSHOP TECHNOLOGY I 



with an optical device to obtain precise setting of the worktable. There are 
no moving parts and no possibility of damage to the measuring devices. 

7.6.4 Modern techniques 

It is now possible to drill and bore holes having their centres to within plus 
and minus two and a half micrometres using a numerical control system. 
Instead of the operator of the jig borer setting the movements of the work- 
table, information is issued in the form of punched holes in cards. This in- 
formation is recorded on tape and fed into a special device which actuates and 
controls the movement of the worktable. The rectangular co-ordinate 
principle is still employed, giving precise movement of the table in the 
direction of the large arrows shown in fig. 7 .9. It is possible to operate a battery 
of jig borers, all performing an identical operation, using one master tape. 
No operators are needed, and the human element is completely eliminated. 

7.6.5 Boring 

Boring a hole is not quite the same thing as drilling a hole. The boring of a 

hole is essentially an opening-out operation, and can only be performed on 

an existing hole. We have seen during our discussion on the centre lathe that 

holes may be bored from the tool post, using a boring bar. In this case the 

& of spindle 




025 



Fig. 7.12.— Principle of the Boring Head. 



HOLE PRODUCTION 



161 



internal cylindrical surface is generated by a combination of work rotation 
and tool feed. 

Much the same technique is used when boring holes at the jig borer, except 
that the boring bar or cutter rotates under feed while the workpiece remains 
stationary. Both techniques, however, have the same aim in view, namely 
the bringing of the hole to a specified diameter and to certain limits of align- 
ment. 

The principle of vertical boring is shown in fig. 7.12. It will be seen that if 
control is to be maintained over the diameter, then precise movement of 
the cutting tool in the direction of arrow X is required. It must be understood 
that the first essential is to ensure that the work centre has been brought truly 
to the spindle centre, for increase of the effective cutting radius shown as R 
affects only the diameter produced; the centre distance is unaffected. 

In the example shown in fig. 7.12 the centre distance between the large hole 
and the two small holes is 158-565 mm. The need for boring is clear, for the 
large hole has a diameter of 1 05 mm. As the diagram shows, the use of a boring 
head permits increase of the radius R but, as pointed out, the worktable must 
be moved a distance of 158565 mm before machining of the large diameter 
is commenced. The example shown is precisely the sort of work that jig borers 
are capable of performing, and it is also possible to carry out a limited amount 
of milling. 

7.7 Horizontal boring 

Horizontal boring may be compared with the technique of the vertical 
boring and turning of large-diameter work dealt with in Chapter 6. 
Horizontal borers, as they are commonly called, are always used for large 
forgings or castings which may require several large bores to be accurately 
machined in line. A typical component is illustrated in fig. 7.13. This is a 




052O 



ALL DIMENSIONS IN MM 

Fig. 7.13. — Typical Component for Horizontal Boring Machine. 



1 62 



WORKSHOP TECHNOLOGY I 



large grey cast iron casting and is to be used as the body of a special-purpose 
gearbox unit. Note the large bores at both the front, rear and one side of the 
casting. The centre line of the 520 mm diameter bores must be at 90 to the 
face X, and also at 90 to the centre lines of the holes C, D and E. Both face X 
and face Y must be truly vertical to the base of the casting. 

If accuracy with respect to the above geometric conditions is to be obtained 
during the machining of this component, it is desirable that as much machin- 
ing as possible be carried out in one setting of the casting. Clearly the opera- 
tions required are boring of the cored holes and machining of the faces X and 
Y. If at the same time it is possible to index the work 90 , then apart from the 
hole diameters, the geometric accuracy of the machining will be equivalent 
to the geometric accuracy inherent in the machine tool used. 

Perhaps fig. 7.14 will serve to illustrate the geometrical requirements of 
the casting shown fully machined in fig. 7.13. The pictorial view shows that 
the axes of the holes are at 90 to each other and perpendicular to the faces 
X and Y. At the same time the axes of the holes must be parallel with the base 
of the casting, with distance Ha. constant value. Added to these somewhat 
exacting geometric requirements, the axes of the three holes must be parallel 
with each other. 

7.7.1 Tooling and feeding on a horizontal borer 

A hole is produced on a horizontal borer using a boring bar. This is shown 
in fig. 7.14. Note that the hole is generated by the feed of the work, as shown 



/ 1 \ 
/ I Boring tool 



Face X 



Facing 
/tool 




Fig. 7.14. — Essential Movements of the Horizontal Boring Machine. 



HOLE PRODUCTION 163 

in the pictorial view. If, now, provision is made for feeding the work at 90 
to the direction of boring, with a single-point cutting tool caused to rotate, 
it is a simple matter to machine or face at precisely 90 to the axis of the bored 
hole. No setting is required from the operator of the machine, merely the 
engagement of the automatic feed actuating the movement of the work- 
table on which the casting is securely clamped. 

Extension of this important principle of making the fullest use of the 
geometric movements built into a machine tool leads to the use of an auxiliary 
table top. This table top may be rotated and indexed in 90 positions, and 
may also be adjusted and set at any other angle. 

Provided an auxiliary table top is used, the casting shown in fig. 7.13 may 
be completely machined in one setting, although it is necessary that the base 
of the casting be first machined and used as a datum face. 

7.7.2 Essential features of the horizontal borer 

Fig. 7.15 illustrates in a simple manner the essential features of a typical 
horizontal borer. The main parts are named and their respective movements 
indicated, and details of the facing head are also shown. With the boring-bar 
holder removed and the facing tool in position, automatic feed of the cross 
slide allows facing of a casting clamped on the auxiliary table. Indexing of 
the auxiliary table top at 90 brings another face ready for machining at 
precisely 90 to the first face. 

Holes may be drilled and bored from the spindle, and for long bored holes 
a long boring bar may be supported by locating in the bearing provided in 
the end column support. In this event the hole is generated by feeding the 
table towards the headstock. Holes of smaller length may be bored by feeding 
an unsupported boring bar held in a suitable holder attached to the facing 
head, generation being achieved by horizontal feed of the spindle. 

It should be clear at this stage that the production of accurate work on a 
horizontal boring machine calls for a very high degree of skill from the crafts- 
man in charge of this expensive and formidable machine tool, designed 
exclusively for the machining of large and heavy castings. 

Snout boring, face milling and surfacing are typical operations possible, 
together with the boring of holes from 15 mm diameter to 500 mm diameter 
to close tolerances. 

Summary 

It is generally accepted that the production of an internal cylindrical 
surface to close dimension tolerance is one of the most difficult of all 
machining operations. It may be remembered that in Chapter 3 mention 
was made of the recommendation in BS 4500 that holes be given a tolerance 
one grade coarser than that given to the shaft which is to mate with them. It 
is because of the difficulty of producing holes to close limits that this recom- 
mendation is made, and this difficulty also accounts for the fact that the 
most expensive general-purpose machine tool is the jig borer. 

We have seen that hole drilling for single or small batch production is 
carried out on either sensitive, pillar or radial drillers, depending on the size 



164 



WORKSHOP TECHNOLOGY I 

v® 



^© 



«M 



^> 




&T<J 



V-© 




DESCRIPTION 


MOVEMENTS 


A 


SPINDLE 


Rotate and feed 


B 


HEADSTOCK 


Vertical, up and down 


C 


COLUMN 




D 


FACING HEAD 


Rotates 


E 


BED 




F 


SADDLE 


Feed along bed 


G 


SUPPORT 


Vertical (with headstock) 


H 


END COLUMN 


Along bed 


J 


AUXILIARY TABLE TOP 


Rotate and index at 90° 


K 


CROSS SLIDE 


Feed at 90° to bed. 



Facing^ ^^ 
too , < L_^ 




Holder for 
boring tool 



FACING HEAD 

Fig. 7.15. — Elements of the Horizontal Boring Machine. 

of the workpiece to be drilled and the diameters of the holes required. On all 
these machines dimensional accuracy with regard to hole centres or the 
linear dimensions of these centres from datum faces is not easy to achieve, 
and when very close or precise dimensional accuracy is required a jig boring 
machine is used. 



HOLE PRODUCTION 165 

The principle of rectangular co-ordinates is widely adopted for the loca- 
tion of hole centres and subsequent drilling or boring, and a sound practical 
working knowledge of trigonometry is a must for the efficient craftsman 
who aims at getting the best out of his machine. 

Modern developments make use of punched cards and tape, whereby 
automatic positioning of the worktable is achieved, eliminating the neces- 
sity for any setting by the operator of the machine. 

When holes are required in components that are to be produced in large 
numbers, three techniques are adopted, depending on the nature and size 
of the component. 

If the part is produced from press tools in relatively thin metal, it is certain 
that the holes will be pierced as part of the pressing operation. When the thick- 
ness of the metal prevents this technique, the principle of drill-guiding may 
be adopted, in which case the dimensional accuracy of the drilled holes is 
obtained by the use of concentric drill bushes accurately located in drill jigs. 

The use of drill jigs may require the application of multiple-spindle drill- 
ing machines arranged in line (sometimes called gang drillers), but if these 
machines are to be worked by a single-operator, then only one spindle at a 
time will be usefully employed in the removal of metal. Thus the most efficient 
method of producing large numbers of holes in a single component consists in 
the use of a multiple-head drilling machine, with upwards of i oo drilling 
spindles engaged in the simultaneous drilling of holes. 

Finally, the production of large-diameter holes demands the use of a hori- 
zontal boring machine, and as the alignment of these bored holes is in respect 
to the machined surfaces from which the holes are located and drilled, the 
design and operation of the horizontal borer provides an interesting example 
in the importance of the best use of the geometric movements built into the 
machine tool. 

Although the principle of broaching was touched upon in a simple manner, 
it must be appreciated that broaching is akin to reaming. It is a hole-finishing 
operation ; a broach, like a reamer, cannot start its own hole, yet for the finish- 
ing of accurate holes on a mass-production basis it has no equal. Although 
perhaps representing the ultimate in the precision engineering of a hardened 
component, resulting in a highly expensive cutting tool, the internal broach 
has opened the way to the precision machining of external surfaces to 
tolerances within plus and minus twenty micrometres, the parts being 
machined on a mass-production basis. 

EXERCISE 7 

1 For the following drilling machines, outline a typical drilling operation, mentioning 
the type of cutting tool used and the appropriate spindle speed: 

(i) sensitive drilling machine, 
(ii) pillar drilling machine, 
(iii) radial drilling machine. 

2 State the advantages of piercing ten 4 mm diameter holes in a component made from 
stainless-steel strip. Make a neat sketch showing the principle involved, and state the limita- 
tions of the piercing process. 



l66 WORKSHOP TECHNOLOGY I 

3 Make a neat sketch of a component requiring the use of a drill jig in order to ensure 
that the part may be mass-produced. Describe two factors that may result in the drilling of 
reject work. 

4 Describe the essential difference between a multi-spindle and a multiple-head drilling 
machine. Which machine performs the drilling operation in the most economical manner? 

5 Describe with the aid of simple sketches the technique of internal broaching as applied 
to the finishing of holes on a mass-production basis. 

6 Make a neat sketch of a component that would require a number of holes to be jig 
bored. With the aid of simple sketches, show how precise linear control of the worktable is 
obtained. 

7 Explain, with a typical example, the necessity for the boring rather than the drilling of 
a hole in an engineering component. Show by means of a neat sketch the geometric movements 
necessary to bore a hole to a precise dimension and to close dimensional tolerances from a 
datum face. 

8 By means of a neat diagram, outline the main features and movements of a horizontal 
boring machine. 

9 Sketch a typical component that would require the use of a horizontal borer for the 
production of large-diameter holes in close alignment to machined surfaces. 

io Show, by means of neat sketches, the device that makes possible the machining of the 
four sides of a large casting at 90 . Show also how a single-point cutting tool may be used to 
machine the surfaces as set up on a horizontal boring machine. 



O Milling Machines 



8.1 Introduction 

In Workshop Processes 2, Chapter 10, we saw that two main types of milling 
machines are in general use : 

(i) horizontal milling machines, 

(ii) vertical milling machines. 

The terms horizontal and vertical refer to the axis of the spindle, for the 
technique underlying the generating or forming of geometric surfaces using 
a milling machine consists of rotation of the cutting tool in conjunction with 
a feed movement of the worktable. Although the vertical milling machine is 
capable of a more versatile range of work, the horizontal machine is preferred 
for heavy cuts. This is because of the greater support and rigidity afforded to 
the milling cutter by the arbor and its support. 

Since the milling cutters used are multi-point (that is to say they have more 
than one cutting point), the rate of metal removal can be very high, and mill- 
ing machines are often to be found on production lines. The range of work 
possible using a horizontal miller is shown in fig. 8.1. 

8.2 Milling operations on the horizontal miller 

8.2.1 Plain milling 

Fig. 8. 1 A illustrates the technique known as plain or slab milling. With 
the work securely held in a rigid fixture, this is a quick and efficient way of 
producing a plane surface parallel to the axis of the cutter. It is suitable as a 
method of machining a first operation to provide a datum face from which 
further machining may be undertaken. 

Slab mills seldom exceed 1 20 mm in length, while the diameter is restricted 
to about 100 mm. Thus the size of the cutter places a limitation on the area 
that can be milled. When fairly large areas require milling the technique of 
face milling is adopted. 

8.2.2 Face milling 

A face-milling cutter generates a surface at 90 to its centre line. The tech- 
nique is illustrated in fig. 8. iB; note that the machined face is at 90 to the 
axis of the cutter. The diameter of face cutters is seldom less than 120 mm ; 
the smaller sizes are often referred to as shell end mills, as described in Work- 
shop Processes 2, Chapter 10. Larger-diameter cutters are of the inserted- 
blade type, with much use of cemented carbide (mentioned in Chapter 4 
of this volume) as the blade material. Note that raising of the worktable 
enables a second cut to be taken, and that because no arbor is used relatively 

167 



1 68 



WORKSHOP TECHNOLOGY I 

Arbor i Slab mill- 




Milled surface parallel 
to t of cutter 



Slab 
milling 

Raising table 
controls depth 
of cut 



Adjust for 

depth of cut x~>. 

^ ^- — ® 

Face 
milling 




© 

Angular 
milling 



Horizontal miller 




Axis of cutter 
tilted 



Vertical miller 



Feed of cutter if required 

Fig. 8.1. — Horizontal and Vertical Milling Techniques, 
large areas can be machined. The close proximity of the facing cutter to the 
headstock of the milling machine promotes good rigidity, an essential re- 
quirement for the efficient removal of a large volume of metal. Remember 
that while the application of slab milling is restricted to horizontal milling 
machines, face milling may be carried out on both horizontal and vertical 
types. 



MILLING MACHINES l6g 

8.2.3 Angular milling 

Angular milling, as the name suggests, consists in the production of a 
milled face at an angle to the centre line on the cutter axis. The technique 
differs slightly, as reference to fig. 8.1C shows, according to whether the 
operation is carried out on a vertical or a horizontal milling machine. Angular 
milling at the horizontal miller can only be achieved with the use of angular 
or form milling cutters. The principle is illustrated in fig. 8.1C. It must be 
remembered that the arbor of a horizontal miller is permanently set at 90 
to the worktable feed ; there is no provision for any movement of the cutter 
other than rotation. The vertical miller, on the other hand, may have the 
cutter axis tilted as shown in fig. 8. 1 C, and it is also possible to feed the cutter 
in a direction parallel with its axis. 

It is important to appreciate that, with the exception of contoured or 
profiled surfaces and the milling of spirals, all milling operations consist of 
one or more of the techniques outlined above. When these surfaces are re- 
quired on a production basis special-purpose milling machines are used, 
the object being to speed up the milling operation, and in general the follow- 
ing techniques are adopted : 

(i) Reduction of non-machining times by provision of rapid traverse of 

the worktable up to the cutting point, with automatic application 

of the correct feed, automatic stop and rapid traverse back to starting 

point. 

(ii) Removal of greater volume of metal by providing greater rigidity of 

set-up and machine tool. 
(iii) Utilisation of two or more cutters. 
(iv) Use of rotary tables. 

8.3 Fixed-bed millers 

Fixed-bed millers are similar in principle to horizontal millers, but are 
larger, heavier and of more rigid construction. The very fact that the bed is 
fixed contributes to this, and it means, of course, that whereas on the hori- 
zontal miller the table can move vertically, on the fixed-bed miller any varia- 
tion in the distance between the cutter centre and the top of the worktable 
must be achieved by adjustment of the cutter slide. This limits the capacity 
of the machine, but even so the machine is eminently suitable for the mass- 
production milling of the medium-sized component. 

Fig. 8.2 illustrates in outline the elements of the fixed-bed-type milling 
machine. Note that only two movements are available apart from the rota- 
tion of the cutter, namely feed of the table and vertical adjustment of the 
cutter spindle. Many of the fixed-bed millers of the type illustrated are pro- 
vided with automatic machining cycles, permitting a predetermined 
sequence of operations to be carried out. 

Thus the operator of the machine merely loads it by placing and clamping 
the workpiece in the milling fixture which is clamped to the table, and then 
presses a button. Immediately the workpiece is fast traversed to the rotating 
cutters, followed by metal removal at a predetermined feed, with automatic 
stopping of the cutters as the work is fast traversed back to the starting point. 



170 



WORKSHOP TECHNOLOGY I 



Workpiece 



Arbor (support not shown) 
J Cutter 




Fast 
traverse 
slow feed 



Straddle milling 

Fig. 8.2. — Fixed-bed Milling Techniques. 

It is possible that the operator may be looking after two or more machines, 
in which case loading or unloading will be taking place at one machine while 
cutting takes place at the other. In this way the milling action may be con- 
sidered as continuous, although two or more machines are in use. 

8.4 Duplex milling machines 

Duplex milling machines make use of the technique of employing two 




Work to We [clamping details not shown] 

Fig. 8.3.— Principle of the Duplex Milling Machine. 



MILLING MACHINES 



171 



cutters in order to increase the efficiency of the milling operations carried 
out. An end view of a duplex milling machine is shown at fig. 8.3. It may be 
appreciated that the type illustrated has a fixed bed. Note the vertical move- 
ments available for the cutter axis, as well as the horizontal adjustment. 
We may consider this machine as a multi-purpose facing machine, very 
suitable for the machining of parallel faces on relatively large castings such 
as machine-tool slides. In special cases a cutter may be in the position shown 
in the diagram by dotted lines ; such a machine would be known as a triplex 
milling machine. 

8.5 Rotary-table production milling 

The technique of rotary-table milling represents perhaps the ultimate in 
production milling. We have seen that the principle of one operator operat- 
ing two or more fixed-bed milling machines tends to produce continuous 

Milling cutters 



Roughing 



Finishing 




Unload 
and load 



Component ready 
for milling 



Tabic rotates 



Fig. 8.4.— Principle of Rotary-table Production Milling. 

milling. This is precisely the aim in making use of a rotary table when milling 
components on a production basis. Fig. 8.4 illustrates the principle involved. 
Note that the rotary table is equipped with identical milling fixtures in which 
the components to be milled are located and clamped. Rotation of the table 
brings each component in turn under the milling cutter, and while removal 
of metal takes place, the operator loads and unloads other components away 
from the rotating cutter. Most rotary-table milling makes use of special- 
purpose machine tools employing two or more milling cutters, the machine 
tools being of very heavy design and construction, giving ample rigidity. 

8.6 Up-cut and down-cut milling 

It is clear from the simple descriptions given that a wide range of milling 
machines is employed for the purpose of milling accurate surfaces on en- 
gineering components ; yet unless the correct type of cutter is used the full 



172 



WORKSHOP TECHNOLOGY I 



advantage of the efficiency of the milling machine will not be obtained. 
Although the more common types of milling cutters were covered in Work- 
shop Processes 2, Chapter 10, no mention was made there of the alternative 
technique known as down-cut milling. 

Up-cut milling is the conventional method of milling, and this technique 
is shown in fig. 8.5A. It will be seen on reference to the diagram that the 
rotation of the cutter is in opposition to the feed of the worktable. Thus the 
tangential force F not only shears the metal but also overcomes the feed force 
of the worktable. At B we see the technique called down-cut or climb 
milling. Note that the rotation of the cutter is with the feed motion of the 
worktable ; there is no tendency for the cutter to oppose this feed motion, 
and thus more power is available for shearing the metal. There are, however, 
other advantages gained by the adoption of the climb-milling technique. 
Reference to fig. 8. 5 A shows that the shear force Facts upwards ; hence there 

down-cut (b) 
(climb) ^-^ 




TABLE FEED 

Fig. 8.5.— Up-cut and Down-cut Milling Technique. 

is a tendency to lift the workpiece offthe table. In fig. 8.5B we see that the shear 
force acts downwards, and this is a much more suitable situation when heavy 
cuts are taken. The cutting force is now taken by the worktable with no 
possibility of vibration or movement; this represents optimum cutting con- 
ditions. 

Fig. 8.6 shows an enlarged view of a single tooth of a milling cutter about 
to take a cut during conventional or up-cut milling. The distance BC repre- 
sents the movement of the table while the tooth of the cutter has rotated 
through the angle a. This distance is much exaggerated in the diagram ; in 
actual practice it amounts only to a few hundredths of a millimetre. Never- 
theless it can be seen that the chip is of zero thickness at the beginning of the 
cut and maximum thickness at the end. It is difficult for the tooth to take a 
bite into the metal, and this results in a considerable area of friction at point A 



MILLING MACHINES 1 73 

in the diagram. Any dulling of the cutting edge of the tooth leads to a marked 
increase in the amount of friction created at the commencement of the cut, 
with consequent decrease in the efficiency of the milling operation. Addi- 
tional power is needed, and the finish of the milled surface deteriorates. 



CUTTER TOOTH 




TABLE FEED 

Start of cut 

Fig. 8.6.— Tooth Action when Up-cut Milling. 

Fig. 8.7 shows an enlarged view of a milling-cutter tooth taking a down-cut 
action. Point A indicates the commencement of the cut, and it will be seen 
that the thickness of the chip removed by the tooth of the milling cutter is at 



CUTTER TOOTH 



Start of cut 
Depth of cut 




TABLE FEED 

Fig. 8.7.— Tooth Action when Down-cut Milling. 



174 WORKSHOP TECHNOLOGY I 

a maximum. Once again the distance AB represents the feed of the table while 
the tooth revolves through the angle a. The cut finishes at the point C with 
zero removal of metal, providing a superior finish to that obtained by con- 
ventional milling. 

Modern milling machines engaged on production milling make much 
use of the climb-milling technique. We have seen that several different types 
of milling machines are available, all calculated to provide maximum 
machining efficiency, in other words maximum metal removal in the mini- 
mum of time. If, however, the climb-milling technique is used, it is essential 
that there be no backlash present in the leadscrew of the milling machine. A 
special device is fitted to the hardened leadscrew which automatically takes 
up the wear between the driving nuts and the leadscrew. This device is 
disengaged if fast traverse is used to bring the work to the cutter, and auto- 
matically re-engaged as soon as the climb-milling cut commences. 

8.7 Negative-rake milling 

We have seen in Chapter 4 that the technique of negative-rake cutting is 
adopted when machining the harder metals at high speeds ; the throw-away 
type of lathe-tool tip used for this purpose was illustrated in fig. 4.9. Apart 
from their great superiority over high-speed steels for cutting the harder 
metals, carbide tips are now available for the cutting of the softer steels, with 
tantalum and titanium as the main alloying elements. The relative brittle- 
ness of these cemented carbides, however, makes the technique of positive- 
rake cutting unsuitable. The main reasons are shown in fig. 8.8 ; the reader is 
also advised to refer back to fig. 4.18, where the advantages of negative-rake 
lathe cutting tools are shown. In both diagrams it will be seen that the main 



Positive rake 




r Negative rake 

1 - © 



Carbide tip 



r Cutter 

U bod y 




Cutting force passes 
outside cutter body 
tip may chip 



Cutting force absorbed 
by cutter body 



Fig. 8.8. — Positive and Negative-rake Milling. 



MILLING MACHINES I 75 

cutting force is absorbed by the body of the tool when the technique of nega- 
tive-rake cutting is adopted. A further advantage is that this cutting force 
acts well up the breast of the tool during negative-rake milling, thus not only 
reducing the risk of chipping of the cutting edge but also materially increas- 
ing the life of the cutting tool. 

Note that at fig. 8.8A the main cutting force is not supported by the body 
of the cutter, resulting in a severe shear plane across the brittle carbide tip. 
At B we see that the effect of the negative rake deflects the cutting force up- 
wards, enabling the cutter body to provide ample support to the carbide tip. 

8.7.1 Negative-rake facing mills 

Best results are obtained when the negative-rake milling cutter is pro- 
vided with a heavy body in which the tungsten- tantalum tips are inserted. 
The heavy mass of the cutter body rotating at high speed tends to have a 



direct,: 




Cutter body v 



Inserted blade 
(3 only shown) 




m= 



X 



Negative axial rake 
I Positive radial rake 

Fig. 8.9. — Inserted-blade Negative-rake Milling Cutter. 



flywheel effect, smoothing out any vibration set up by the intermittent 
cutting action of the blades. The elimination of torsional vibrations pro- 
motes a good finish to the milled surface, with complete absence of the wavy 
effect more commonly known as chatter. 

Fig. 8.9 shows, in diagrammatic form, a simple inserted-blade negative- 
rake milling cutter. Note that the cutter shown possesses both axial and 
radial rake. It is customary to keep the diameter of the milling cutter greater 
than the width of the workpiece, as shown in fig. 8. 10A. The diagram shows a 
plan view of the milling operation, with one blade or inserted tooth about to 
take a cut. Note that the blade makes an angle of 45 ° as it contacts the work- 
piece. At B we see a front view of the inserted blade ; the shaded area represents 
the portion of the cutter subject to the severe frictional forces of the fast- 
moving chip. 



I76 WORKSHOP TECHNOLOGY I 

8.7.2 Further advantages of negative-rake milling 

Reference back to fig. 8.8 shows that the chip is relatively scraped or 
struck away from the workpiece owing to the negative rake on the cutting 
blade. All negative-rake milling is carried out using very high spindle speeds, 
and this means that considerable friction is created at the area of contact. 
This friction leads to a sudden rise in temperature, lowering the shear stress 
of the metal so that less power is required to shear the chip. The high speeds 
employed result in rapid removal of the chip ; thus the heat generated is 
carried away with little or no heating of the workpiece. A very good surface 
finish results from negative-rake milling, provided a rigid milling machine is 
used and ample power is available. It is also advised that the feed per tooth 
exceeds o-i mm; up to 03 mm per tooth gives excellent results because 
rubbing of the teeth is prevented. 

Area of metal removed per blade 




Clearance 
angle 

Front view of blade 



FEED 

Plan view of 
cutting action 

Fig. 8.10.— Cutting Action of Negative-rake Milling Cutter. 
8.8 Form milling 

Form-milling cutters are used to produce profiled surfaces. A good 
example of the need for form milling is provided when the top surfaces of the 
components shown in fig. 8.1 1 are to be produced using milling machines. 
It will be seen that the profile of the surface shown at A is a combination of 
plane and internal and external cylindrical surfaces, and that these surfaces 
are produced by reproduction of the surfaces of the milling cutters used. 

At B we see a rack with which the spur gear shown at C is to mesh. Although 
the teeth of the rack possess straight sides or flanks, those of the spur gear 
have the popular involute form. A special milling cutter is needed for the 
milling of the involute teeth, and it is an essential condition of this cutter that 



MILLING MACHINES 177 

it continues to reproduce an accurate form after repeated sharpenings. 
The accuracy of the involute form is important if the spur gear is to mesh 
efficiently with the rack; to obtain this accuracy the cutter is form-relieved. 

8.8.1 Form-relieved milling cutters 

A form-relieved milling cutter is so machined that, provided the cutter is 
sharpened by grinding the front faces of the teeth, no change of the tooth con- 
tour takes place. Fig. 8.12 illustrates the principle involved; the diagram 
shows a sectional front elevation of the involute gear cutter used to mill the 
teeth of the spur gear shown in fig. 8. 1 1 C. 

The teeth have their front faces lying on a radial plane, as shown in the 
diagram by the lines oa, ob, oc and od. In the enlarged view of a tooth it will 




Fig. 8.1 1.— Applications of Form-relieved Milling Cutters. 

be seen that, provided a radial plane is taken, the contour of the tooth remains 
unchanged. Special form-relieving devices are required for machining the 
relief on the cutting teeth, and particular care is required when regrinding 
the cutter to ensure that each tooth is accurately reground on a true radial 
plane. Form cutters similar to the cutter described above give best results 
when the cutting faces are brought to a high degree of finish. 

8.9 Universal wiilHwg machine 

The universal milling machine may be considered as a combination of 
both horizontal and vertical types, with provision for adjustment of the line 
of action or feeding direction of the worktable. The increased tool and table 
movements make possible a wide range of milling operations not practicable 



1 7 8 



WORKSHOP TECHNOLOGY I 

b 




Fig. 8.12. — Principle of the Form-relieved Milling Cutter. 

Drive transmitted 
through 90° 




End mill 




Shell end mill 




Tee slot cutter 



Slotting cutter 



Pi I 

Fig. 8.13.— Vertical-milling Attachment. 



MILLING MACHINES 

with a straight vertical- or horizontal 
is a horizontal miller, but the followi 



79 



1 a straight vertical- or horizontal-type machine. Basically the machine 
horizontal miller, but the following attachments are available : 



8.9.1 Vertical-milling attachment 

This is shown in fig. 8.13, together with the milling cutters and typical 
operations possible with these cutters. The attachment may be considered 
as a 90 gearbox device with a ratio of 1 : 1 . When small-diameter milling 

-Slotting attachment 




i_K 




^'Clearance 
,v : -,angle 



Details of cutting 
tool 



Paralle l ba r— 



^ %y/A Ij pzfi 



§ 



-Clamps 



Milling m/c table 




Cross slide feed 



Die insert 

Fig. 8.14. — Slotting Attachment. 



Table feed 



i8o 



WORKSHOP TECHNOLOGY I 



cutters are to be used, a high-speed milling attachment may be fitted to the 
machine instead of the standard-type vertical milling attachment. The high- 
speed attachment has a speed increase of about 3:1, permitting a wide range 
of spindle speeds. 

8.9.2 Slotting attachment 

The purpose of the slotting attachment for a vertical milling machine is to 
convert the rotary motion of the spindle into reciprocating motion ; in this 
way it is possible to machine slots and keyways. The slotting head is capable of 
swivelling to any desired angle, and the length of stroke is about 1 00 mm. Fig. 
8.14 shows a slotting attachment together with an example of its use. Note 
the rake and clearance angles on the slotting tool as shown in the diagram, 
and the use of the table and cross-slide feed to machine the sides of the die 
openings at 90 . Because the slotting tool must clear the work at the end of 
the cutting stroke it is necessary to clamp the work on two parallel bars, 
as shown in the diagram. 

8.9.3 The dividing head 

The dividing head is a most important attachment of the universal milling 
machine. As its name suggests, its main purpose is the location and holding of 
work in order that machining may take place by suitable division of the work- 
piece. The gear shown in fig. 8.1 iC is a good example of a component that 
requires the use of a dividing head, for if 48 teeth are required the work must 
be indexed 48 times. Before dealing with the different methods of indexing 
it is as well to remember that the dividing head is a most versatile work-hold- 
ing device, capable of holding cylindrical work with the axis horizontal, 
vertical, or at any angle between the horizontal and the vertical ; the work 

j^^Milling cutter 




Axis vertical 
component held in chuck 





Axis horizontal, 

component held between 

centres 



Axis at an angle 
component held in chuck 



Fig. 8.15.— Axis Positions Possible with Dividing Head. 



MILLING MACHINES 



181 



can be chucked or held between centres. Three simple examples are shown 
in fig. 8.15. 

8.10 Methods of indexing 

Four methods of indexing are in use : 

(i) direct indexing, 

(ii) simple indexing, 
(iii) compound indexing, 
(iv) differential indexing. 

8. 10. 1 Direct indexing 

An example of direct indexing is given in Workshop Processes 2, Chapter 10, 
where the technique is shown in figs. 1 74 and 1 75. It will be seen that the prin- 
ciple of direct indexing makes use of a gear or disc having the required 
divisions or slots into which a locating device engages. The indexing is 
directly obtained; there are no gearing arrangements. 

8.10.2 Simple indexing 

Simple indexing is always carried out using a standard 40:1 dividing 



Spindle in horizontal 
position 



Crank 

Hole circle plate 

Work spindle 




Worm wheel 

fork spindle 

Spring loaded plunger 
Hole circle I r-i 




r Handle 



Worm 



Fig. 8.16.— Principle of Simple Indexing. 



l82 WORKSHOP TECHNOLOGY I 

head. An external view of a dividing head is given in fig. 8.16A. Note that 
the work-holding spindle is in the horizontal position, but can be tilted in the 
direction of arrow Y to the vertical position. The means by which rotation of 
the work spindle is achieved is shown in fig. 8. 1 6B. Rotation of the crank turns 
the worm spindle which carries a worm meshing with the worm wheel. In 
order to rotate the work spindle one complete revolution the worm spindle 
requires rotating 40 times; that is to say 40 turns of the crank are required. 
One turn of the crank rotates the work spindle one-fortieth of a revolution ; 
thus if a 40-tooth gear requires to be milled the simple indexing for this gear 
consists of one turn of the crank. 

The following formula may be used to determine the indexing necessary 
when milling slots or teeth : 

Number of turns on crank = %-? 

where JV = number of division required. 

Clearly, provided the number of divisions to be milled is a factor of 40, for 
example 2, 4, 5, 8, 10 and 20, the indexing is easily achieved using the formula 
given above, the solution representing complete turns of the crank. 

Thus if 5 equi-spaced slots are to be milled on a steel blank, the indexing 
required is simply calculated as follows : 



Number of turns on crank = 



_ 4° 

~ff 

_ 4° 

~ T 

= 8 turns of the crank. 

8.10.3 Use of hole-circle plates 

Hole-circle plates are needed when the number of turns of the crank has a 
fractional value. Let us use the formula to calculate the indexing to mill the 
48-tooth gear shown in fig. 8.1 iC. 

Number of turns on crank = %q 

_ 4° 

~ 48 

= ^ turn of the crank, 
o 

This is a fractional indexing, and a device is needed that will permit not only 
five-sixths of a turn of the crank, but also a very wide range of other possible 
fractional indexings. 

Fig. 8. 1 7 shows the device known as a hole-circle plate. The one illustrated 
is a standard plate used on the Cincinnati dividing head. This plate is revers- 
ible, having on one side the following hole circles : 

24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43 holes 
and on the reverse the following: 

46, 47 > 49> 5 J > 53, 54, 57. 5®, 59> 62 > 66 holes 
Reference to fig. 8.17 shows that the plunger which is attached to the crank 



MILLING MACHINES 183 

has been adjusted to locate in the 24-hole circle, and the indexing is now 20 
holes on the 24-hole circle, because § is equal to f£. 

With eleven hole circles on either side of the plate it is a simple matter to 
adjust the spring-loaded plunger so that it will locate in any required hole 
circle, and all divisions up to 60 can be obtained by using this reversible plate. 
Gears may be cut with teeth of all even numbers and multiples of 5 from 10 
to 1 20. Perhaps two examples will show the great versatility of the Cincinnati 
reversible hole-circle plate. 



SECOND PLUNGE 




FIRST PLUNGE 

ADJUSTABLE ARMS 



INDEXING- ~" 
o 



«2Q 
24 



^9gX 2Q.. 



"/*/ Ho ' e c ' rc 'e ptate 

^<c^^ /ONLY 24 HOLE CIRCLE'N 



ISHOWN 



Fig. 8.17.— Application of Hole-ciFcle Plate. 
EXAMPLES 

(i) Calculate the indexing required to mill a 59-tooth involute gear using 
a Cincinnati universal milling machine. 

Number of turns on crank = %- r 

= 40 
59 

Indexing required is 40 holes on a 59-hole circle. As a precaution it is a wise 
policy to make full use of the quadrant shown in fig. 8.17. Both arms of this 
quadrant are freely adjustable, and when set to the required indexing re- 
move the necessity for counting out the holes after each milling cut, and per- 
haps more important, reduce the possibility of scrapping the work through 
miscounting the number of holes. 

(ii) Calculate the indexing required to mill eleven equi-spaced slots on 
a turned blank. 



1 84 



WORKSHOP TECHNOLOGY I 



Number of turns on crank = %r 



_ 40 
1 1 



The indexing is 3 whole turns and rr of a turn. We do not have an 1 i-hole 
circle, but 

_7_ _ 42 
11 ~ 66 

therefore the indexing is 3 whole turns plus 42 holes on a 66-hole circle. 

8.10.4 Angular indexing 

The need for angular indexing may be appreciated by reference to fig. 
8.18. The component shown is a mild-steel turned blank, and it will be seen 
that three slots require to be machined. The first slot is to be machined with 
its centre coinciding with the centre line of the component, while the second 
is to be milled with its centre at an angle of 90 to the first slot. The third slot 
is to have its centre at 1 19 30' to the second slot. 



1st slot 




Fig. 8.18.— Component Requiring Milled Slots. 
We have seen that one complete turn of the crank results in jq of a turn of 
the work spindle, to which the component is attached using either a chuck 
or centres. Expressed in terms of angular notation, ^j of a turn is equal to 
9 , and the following formula may be used in order to calculate the indexing 
required when machining slots to angular dimensions: 

Number of turns on crank = — 

9 

where A = angle required in degrees. 



MILLING MACHINES 185 

Thus indexing for the second slot may be calculated as follows: 

Number of turns on crank = — 

9 

= 2° 
9 
= 10 
To calculate the indexing for the third slot : 

Number of turns on crank = — 

9 

= 2J9i 

9 
_ 239 

~ 18 

= i3u$ = 1354 

The indexing for the component is now given below : 

(i) Set component on centre, adjust plunger to 54-hole circle, remove 
backlash and engage in zero hole; mill first slot. 

(ii) Index 10 complete turns of crank and engage plunger in zero hole; 
mill slot. 

(iii) Index 13 complete turns plus 15 holes on 54-hole circle, engage 
plunger; mill slot. 

8.10.5 Compound indexing 

This technique is used when the dividing head is fitted with two plungers, 
as shown in the diagram in fig. 8.19, and the divisions required cannot be 
obtained by simple indexing. Reference to the diagram shows that two 
plungers are available, one on the turning crank and the other on a fixed 
pivot. The crank is rotated in the usual way, followed by rotation of the hole 
circle or index plate. During rotation or indexing of the crank the fixed 
plunger is engaged in a hole in the plate, thus holding the plate firmly in 
position. With the first indexing completed, the plate and the indexed 
crank are further rotated either in the same direction as the crank indexing 
or in the opposite direction. 

Let the crank indexing bea holes on an ^4-hole circle, and the plate indexing 
b holes on a l?-hole circle in the same direction. 

Then the total indexing is — +— . 

A B 

If, on the other hand, the plate is rotated in the opposite direction to the 

crank, then the total indexing is 4 — -5. 

Thus the object of compound indexing is to calculate two fractional values 
whose sum or difference is equal to the actual indexing fraction required. 
Let us assume that a 91 -tooth involute gear requires milling of the teeth. 
Using the formula 

Turns of crank = \- r 

N 



1 86 



WORKSHOP TECHNOLOGY I 



the required fraction is f£. 

Checking the hole circles available on the index plate we find that a 9 1 -hole 
circle is not among them. This is therefore an indexing operation that cannot 
be carried out using the simple indexing technique. 

We will use the Brown and Sharpe dividing head for the compound in- 
dexing of the 9 1 -tooth gear, choosing suitable hole-circle plates from one 
of the three available having the hole circles given below : 

Plate 1 : 15, 16, 17, 18, 19, 20 holes 

Plate 2: 21, 23, 27, 29, 31, 33 holes 

Plate 3: 37, 39, 41, 43, 47, 49 holes 



Crank indexed 
Hole circle A 



in Hole Circle A 




Hole circle plate indexed 
-b holes in Hole Circle B 



INDEXING 



a holes . b holes 



Hole circle A Hole circle B 

Fig. 8.19.— Principle of Compound Indexing. 

Method of calculating compound indexing 

First reverse the fractional indexing required and write it down thus : 

91 
40 

Now factorise both numbers. 

9i = 7X13 



40 = 5x2x2x2 



The next step is to choose two hole circles from the same plate ; write down 
the relevant numbers and their factors below the line, and their difference 
with its factors above the line. If all the factors above the line cancel out the 
two hole circles chosen are suitable. Clearly the factors 13 and 7 are needed 
below the line ; Plate 3 is an obvious choice, as it has the hole circles of 39 
(13x3) and 49 (7x7). 



MILLING MACHINES 187 

Inserting these numbers below the line and their differences above the 
line, 

10 = £x£ 
91 = t* H 



40 = & X £ X 2 X 2 

39 = 3 x tt 
49 = i x 7 

Note that all the numbers above the line cancel out; this means that the hole 
circles of 39 and 49 may be used, as shown in fig. 8.19. The problem now is to 
determine the number of holes to be indexed on each hole circle. 
Writing down the two fractional indexings, we have : 
a holes , b holes 40 



39- hole circle ~ 49-hole circle 9 1 

Note that plus means in the same direction as the crank indexing, minus in 
the opposite direction (see fig. 8.19). 

From *- ± i- = £ 

39 49 9 1 

a . b _ 40 



(i3 x 3)~(7X7) (i3 x 7) 

49^ ± 396 = 40x21 
13x7x7x3 13x7x7x3 

49 fl ± 39* = 8 4° 

It is now necessary to find values for both a and b that will satisfy the equation 
above, and the method of trial and error must be adopted. 
When a = 6 and b = 14, 

49x6 + 39x14 = 294 + 546 = 840 
The compound indexing for the milling of the 91 -tooth gear comprises 
the two following indexings : 

(i) 6 holes on a 39-hole circle with the crank, 
(ii) 14 holes on a 49-hole circle, moving or indexing the plate in the same 

direction as the crank. 
It may be appreciated that the calculations involved in compound in- 
dexing have not made this technique a popular system, and it must be further 
understood that the method is only possible when the dividing head is 
equipped with two plungers. A more popular method is the technique known 
as differential indexing. 

8.10.6 Differential indexing 

We have seen that the principle underlying compound indexing consists 
in making use of the addition or subtraction of two fractions, the nett result 
being equivalent to the actual fraction required. One fractional indexing 
is made by the crank, while the other rotates the index or hole-circle plate. 
Much the same principle is used in differential indexing, except that rotation 



l88 WORKSHOP TECHNOLOGY I 

or indexing of the index plate is achieved through a suitable gearing arrange- 
ment. 

Fig. 8.20 illustrates the essential movements of a dividing head set up for 
differential indexing. Rotation of the spindle crank causes rotation of the 
work spindle through the worm and worm wheel in the ratio of 40 : i . Refer- 
ence to the plan view shows that the simple gear train comprising the spur 
gears A, B and C transmits rotary motion from the work spindle through two 
equal bevel gears and two equal spur gears to the index plate. Thus rotation 
of the crank causes rotation of the index plate, the direction of rotation being 
determined by the insertion or removal of an idler gear in the gear train. In 
this way two fractional movements may be obtained ; one by indexing the 
crank by hand in the usual way, and the other by the choice and insertion of 
suitable gears to give the required movement of the index plate. 

Method of differential indexing 

In dealing with the method of calculating a suitable gear train for differ- 
ential indexing, we will assume that the Brown and Sharpe dividing head is 
to be used. This means that we must know the number and tooth range of 
the gears. These are given below : 

20 (2), 24, 28, 32, 40, 44, 48, 56, 64, 72, 86, 100 teeth. 

Let us assume that we wish to mill a gear of 97 teeth using an involute 
form cutter. 

Number of turns on crank = %? = — 

N 97 

Thus if a 97-hole circle were available this fractional indexing would be 
carried out 97 times, at the end of which the work spindle would have made 
one complete revolution and all the teeth of the gear would have been 
machined. 

Then — X07 = 40 turns of the crank. 

97 y/ * 

In order to calculate the gears required it is first necessary to make a close 
approximation to the ratio — , and clearly if this approximate ratio is in- 
dexed 97 times an error results. The work will not make a complete revolu- 
tion; if the approximation is less than — a full turn is not made, and if it is 

more than — the work revolves in excess of a full turn. The purpose of the 
gearing is to counteract the effect of the approximation. 



100 



Choosing an approximation of -^ 

40 
97 



— = -2— (approximately) 



_8_ 
20 



MILLING MACHINES 



l8 9 



We may attach the no. 1 plate to the dividing head and index 8 holes on a 
20-hole circle. After milling 97 teeth the total indexing is 

— XQ7 = 23— = 38-HJ turns of the crank. 
20 *' 20 ° 1U 




Plunger out 



-Crank 



Worm 



Crank spindle rotates 
Work spindle 




Hole circle plate 
and gear free to 
rotate on crank 
I spindle 



B 
IDLER 



1st driver 



1st driven 
4OT 




I spur gears 
Equal bevel gears 

Intermediate spindle 
C 
DRIVEN 



2nd driven 
24T 



2nd driver 
24T 



Fig. 8.20.— Gearing Arrangement for Differential Indexing. 



19° WORKSHOP TECHNOLOGY I 

Clearly this is i^ turns short, and gears must now be chosen in the follow- 
ing ratio : 

Drivers _ 12 _ 6 
Driven 10 5 

2 3 

1 5 
= 48 x 24 
24 40 

Thus the driving gears must have 48 and 24, and the driven gears 24 and 40 
teeth respectively. The train needed for this gearing arrangement is shown 
in fig. 8.20B; the index plate is to turn in the same direction as the crank. 

With this gearing arrangement set up, the crank is indexed 8 holes on a 
20-hole circle, an imperceptible movement of the index plate taking place 
in the same direction as the crank. 

8. 1 1 Spiral milling 

Spiral milling provides yet another example of the versatility of the univer- 
sal milling machine when equipped with a standard 40:1 dividing head 
In simple, compound and differential indexing rotation of the crank is 
carried out by hand ; there is no automatic feed or drive through the lead- 
screw of the milling machine. For spiral milling it is essential that the worm 
spindle (see fig. 8.16A) be geared to the leadscrew of the milling machine. 

A typical set-up for spiral milling is shown in fig. 8.21, where it will be seen 
, that the worm spindle is geared to the leadscrew of the milling machine. With 
the traverse of the table feed engaged the rotating leadscrew drives the worm 
spindle, which in turn drives the worm wheel £nd hence the workpiece. The 
index plate (together with the crank) rotates also, and as in differential in- 
dexing the index plate must be unlocked. 

8. 1 1. 1 Lead of the machine 

Before any spiral milling is carried out the lead of the machine must be 
known. This is the lead produced when equal gears are used between the mil- 
ling machine leadscrew and the worm spindle, as shown in fig. 8.21. If the 
workpiece is to make one complete revolution, 40 turns of the worm spindle 
are required and 40 turns of the milling machine leadscrew. Most milling 
machines have leadscrews of 5 mm pitch ; thus 40 turns of the leadscrew pro- 
duce a linear movement of the table equal to 4 x 40 = 200 mm. 

Special milling machines have different leadscrews, and it is also possible 
to obtain dividing heads with a ratio of 80 : 1 . The formula for calculating the 
lead of any machine is : 

Lead of machine Rx P 
where R = ratio of dividing head 
P = pitch of leadscrew 

EXAMPLE 

Calculate the lead of a milling machine having a 5 mm pitch leadscrew and 
using an 80:1 -ratio dividing head. 



Supported by tailstock 



MILLING MACHINES 

Arbor 



191 



Hole circle plate rotates 
ratio 




2QO mm Lead 




FEED Intermediate spindle 



5mm pitch leadscrew 
rotates 40 times 



Helix angle 



Fig. 8.21. — Set-up for Spiral Milling. 



Lead of machine = RxP 

= 80x5 

= 400 mm 

Using the conversion factor of 1 in = 25-4 mm, 

400 in 
400 mm = - 



= 15-748 in (to the nearest 7000 in) 



8.1 1.2 Calculating the spiral angle 

The spiral angle is the angle made by the machined flute or form with 
respect to the centre line of the workpiece. A more correct name for this angle 
is the helix angle, and it will be seen from fig. 8.22 that if the spiral milling is 
to be performed on a horizontal milling machine the table must be swivelled 
in order to bring the cutter in line with the machined form. All universal 
milling machines have this ability to swivel the worktable through a small 
arc, usually about 30 . 



192 



WORKSHOP TECHNOLOGY I 



ARBOR CUTTER 




FEED 
DIVIDING HEAD 



8 flutes 300mm lead 04O 




300 mm LEAD 

Fig. 8.22.— Need for Angular Setting of Table when Spiral Milling. 



When a spiral is to be milled the following information must be available : 
(i) lead required, 
(ii) lead of machine, 
(hi) spiral angle, 
(iv) mean diameter of work. 

The formula given below may be used to calculate the spiral angle : 
_ mean circumference of cylinder 
lead of helix 
where a = spiral angle 



EXAMPLE 

A milling cutter of 40 mm diameter and 300 mm lead, with 8 flutes of 6-5 
mm depth, is to be gashed (milled) using a universal milling machine. Cal- 
culate the angular setting for the table. 

This cutter is illustrated in fig. 8.22, together with a development showing 
the derivation of the above formula. 



MILLING MACHINES 



193 



With 



_ mean circumference of cylinder 
— lead of helix 

= *R. 
300 



= 22 x 335 
7 300 
(Note: mean diameter = outside dia. — depth of flute) 

= 737 
2100 

= 03509 

Hence spiral angle a = 19 20'. 

With the two locking nuts slackened off, the milling-machine table is 
swivelled until the angle of 19 20' is indexed at the zero mark; unless the 
circular scale is fitted with a vernier device the 20 minutes of arc or ^° must 
be estimated. 

8. 1 1.3 Calculating the gear train 

Let us assume that the milling cutter illustrated in fig. 8.22 is to be gashed 
using a standard 40 : 1 dividing head on a metric machine with a leadscrew 
of 5 mm pitch. 

The formula required to calculate the correct gears between the milling- 
machine leadscrew and the worm spindle of the dividing head is as follows: 
Drivers lead of machine 



Driven 



lead to be cut 



Index -five turns of crank 

Plate rotates during milling of flutes 



r 72 Driven 



Work spindle 




Idler 



Driver 



-^Leadscrew 



Fig. 8.23. — Gearing Arrangements when Spiral Milling. 



194 WORKSHOP TECHNOLOGY I 

Using this formula in order to calculate the gears for the gashing of our 
300 mm lead milling cutter : 

Drivers _ 200 
Driven 300 
_ 2 
~ 3 

72 

The gears available are those given for differential indexing, dealt with 
earlier (8.10.6). 

The indexing for the eight flutes of the cutter proceeds in the same manner 
as simple indexing. For the first cut, the plunger on the crank is engaged in 
the zero hole of the 24-hole circle. With the table traverse engaged the index 
plate and the crank rotate; a table stop disengages the traverse, allowing 
the table to be brought back by hand to the starting position. The crank is 
now indexed five whole turns, the plunger firmly engaged and the milling 
operation repeated. 

Note the insertion of two idler gears between the milling-machine lead- 
screw and the worm spindle; the set-up is shown in fig. 8.23. 

8.12 Cam milling 

The milling of the contour of a constant-rise cam is a relatively simple 
affair, provided a universal miller, a 40 : 1 dividing head and the appropriate 
formula are available. Before dealing with the actual machining of the cam 
it may be as well to have a quick look at the construction and use of constant- 
rise cams. Fig. 8.24 shows a simple cam ; note that from A to D the profile of 
the cam rises. In other words, if a roller rests at A, then rotation of the cam 
through 1 8o° produces a linear movement of the roller from A to B. This 
principle of converting rotary motion into linear motion is much used when 
machining with automatic lathes; the movement of the cutting tool is deter- 
mined by the action of a cam on a roller. In turn, the number of revolutions 
of the work determines the amount of revolution of the cam. 

8.12. 1 Lead of a cam 

This is the linear rise or fall if the contour of the cam is taken through 
360 ; the calculation required to determine the lead of any cam is a matter 
of simple proportion. 

In the diagram in fig. 8.24 the rise of the cam is given as 50 mm. 
Thus 

Rise in 180 = 50 mm 

Rise in 360 = 50 x ^^- mm 

= 100 mm 

8.12.2 Set-up for cam milling 

As already stated, cam milling is only possible provided a vertical milling 



MILLING MACHINES 



*95 



head is available and this head is capable of rotary adjustment. We have seen 
that the axis of the work spindle of a dividing head may be set at any angle, 
and fig. 8.24B shows an extreme case, with both the cutter spindle and work 
spindle in a vertical position. If, now, the worm spindle of the dividing head 
is geared to the leadscrew of the milling machine, and automatic traverse of 
the table engaged, the rotation of the cam blank in conjunction with the table 
feed results in the machining of a constant-rise contour. 

Another extreme case is shown in fig. 8.24C ; here both the cutter and work 

Linear rise in 
^~I80° »50mm 




Fig. 8.24.— Principle of Cam Milling, 
axis are in the horizontal position. With the dividing head still geared to the 
milling-machine leadscrew and the traverse engaged, a cylindrical surface 
is machined. The distance shown as R remains constant. 

It may now be appreciated that tilting of the cutter and work axis and 
control of the rate of table feed allow the milling of constant-rise cams as 
shown in the set-up in fig. 8.25. The angle of tilt of both the work and spindle 
axis is shown as a in the diagram ; note the use of compound gearing between 
the milling-machine leadscrew and the worm spindle. 



19^ WORKSHOP TECHNOLOGY I 

8.12.3 Calculations for cam milling 

The following symbols are used in calculations for cam milling, and the 
reader should become familiar with them : 

a = angle of inclination, 
R = gear ratio, 
L = lead of cam, 
p = pitch of leadscrew. 
Provided a metric milling machine with a leadscrew of 5 mm pitch is used, 
we may employ the formula 



R = 



200 sin a 



which may be expressed in full as 

n . _ 200 sin angle of inclination 

Lxear ratio — i — , — j; 

lead of cam 

In the event of a British milling machine being used, the formula will be 

„ _ 40 sin axp 

A 40:1 dividing head is used, and the metric pitch and the lead to be milled 
are converted to inches. 



EXAMPLE 

The following example will give an indication of the use of the formula 
given above when cam milling on the universal milling machine. 



Cutter 




Intermediate spindle Leadscrew 

Fig. 8.25.— Set-up for Cam Milling. 



MIL1.ING MACHINES 197 

Calculate the gearing and angular inclination for milling a cam having a 
constant rise of 40 mm in 1 50 of its angle, using a standard 40 : 1 dividing head 
and a milling machine with a leadscrew of 5 mm pitch. 

From the formula 

D 200 sin a 
it will be seen that there are three unknown values, R, a and L. 
10 find L Profile rise in 150 = 40 mm 

Profile rise in 1 ° = -^— 
150 

Profile rise in *6o° = * 3 — 

150 

= 96 mm = lead of cam = L. 
With L known, two unknown values remain, and it is now necessary to 
assume one and calculate the other. This means that the gear ratio R may be 
assumed to be, say, 3 :2, and this may be inserted in the formula and a 
calculated. 

Thus 



R 

3 


= 


200 sin a 


200 sin a 


2 
sin a 


96 

96x3 
2X 200 
072 



The angular inclination is therefore equal to the angle whose sine is 072, 
and from tables we find this to be 46 4'. Many dividing heads are provided 
with setting pins at 100 mm centres, allowing the use of a sine bar for precise 
angular setting of the work-spindle axis. 

Reference back to fig. 8.25 shows that as the cut proceeds the work moves 
up the cutter, and this means that a reasonably long cutter is needed. At 
the same time, if the contour is to be milled from a circular blank the depth 
of cut increases as the table traverses. This is a most unsatisfactory arrange- 
ment, and the correct procedure is to rough out the profile first with a band- 
saw, and then set up for milling. In this way the amount of metal removed 
will be reasonably constant, and if roughing and finishing cuts are taken a 
well-finished, accurate cam will result. 

The accuracy may be readily checked by mounting the cam between 
centres on a suitable mandrel, and checking the linear rise using end stand- 
ards in conjunction with a dial indicator. 

Summary 

The production of plane and contoured surfaces by the milling process is 

an established production technique, although there is, as we shall see in a 

later volume, a tendency for the surface broaching technique to replace 

face milling as a method of producing relatively large plane or contoured 



I98 WORKSHOP TECHNOLOGY I 

surfaces. We have seen in this chapter that milling may be generally divided 
into two main classes : 
(i) production milling, 

(ii) small batch or specialised milling. 

When production milling the accent is on maximum metal removal to- 
gether with close dimensional accuracy and good surface finish. For this 
kind of work the milling machines must be of rigid construction with ample 
power available ; this allows the technique of negative-rake milling to be 
adopted. 

When maximum output is required the method used is that of rotary 
milling, in which case the removal of metal is virtually continuous. Profiled 
surfaces are readily produced using form cutters, which require sharpening 
on the front faces of the teeth. Finally the universal milling machine, with the 
attachments available, provides an extremely versatile range of machining 
set-ups. These include simple, compound and differential indexing and 
spiral and cam milling. To produce accurate, well-finished results from all 
of these operations the mechanical engineering technician must know and 
be able to manipulate not only the milling machine but also the necessary 
mathematical formulae. 



EXERCISE 8 

i With a neat diagram, illustrate the principle of the milling operation and explain the 
importance of this technique in precision engineering manufacture. 

2 Illustrate with typical examples the following milling techniques : 

(i) face milling, 
(ii) angular milling, 
(iii) vertical milling, 
(iv) horizontal milling. 

3 Outline the differences in construction between a standard horizontal milling machine 
and a miller designed for high-production horizontal milling. 

4 Explain what is meant by the technique of rotary-table milling. Sketch a component 
for which this milling technique would be suitable. 

5 What is the difference between up-cut and down-cut milling? Define the requirements 
if a plane surface is to be produced by the down-cut milling technique. 

6 Explain the difference in cutter design when considering the manufacture of milling 
cutters for both positive- and negative-rake milling. 

7 Sketch a component that would require the application of a form-relieved milling 
cutter. What precautions must be taken when regrinding a form cutter? Why must the cutter 
be kept always in a sharp condition ? 

8 A gear is to have 1 27 teeth of involute form for the cutting of metric threads at a centre 
lathe. If a Brown and Sharpe 40 : 1 dividing head is available, calculate a suitable set-up for 
the differential indexing of this gear when gashing the teeth using a universal milling machine. 

9 Make the necessary calculations for milling 10 flutes of 5 mm depth, having a rake angle 
of 1 8° and a lead of 250 mm. The spiral angle of the flutes is 12 . Also make a neat sketch of 
the set-up. 

10 Calculate and illustrate a suitable setting for the milling of a constant-rise cam whose 
profile rises 15 mm in 120 of its angle. A standard 40:1 dividing head is available of Brown 
and Sharpe design, together with a universal milling machine of 5 mm pitch leadscrew. 



Index 

Numbers in bold type refer to illustrations 



Airy points, 28, 2.3 
Angular indexing, 184-5, 8.18 
Angular milling, 169,8.1c 
Arc welding 9-13, 1.10-1.14 
Argon arc welding, 12-13, 1.14 

BS 969, 74-7 

BS 1044, 69-73 

BS 4500, 56-69 

Balancing of faceplate, 106-8, 5.8 

Balls, precision, 28, 2.8 

Bare wire electrode, 10, 12-13, 1.11, 1.13, 

1.14B 
Boring 

horizontal, 161-3, 7.13-7.14 

vertical, 117-20, 5.19-5.20 
Boring bars, 132, 6.9 
Broaching of holes, 155-6, 7.7 
Built-up edge, 89-91, 4.13-4.14 
Butt welding, 18, 1.3, 1.20 
Button boring, 102-6, 5.3-5.7 

Cam milling, 194-7, 8.24-8.25 
Capstan lathes, 123-46, 6.1-6.22 
Cemented carbides, 83-6, 4.7-4.8, 4.10 
Centre lathes, 99-122, 5.1-5.21 
Centres, 109-10, 5.10-5.11 
Ceramic-tipped tools, 84, 86-7, 4.8-4.10 
Chucks 

capstan and turret lathes, 1 28, 6.7B 

centre lathes, 100-2, 5.2 
Clearance fits, 63-4, 3.7 
Climb milling, 17 1-4, 8.5, 8.7 
Collets 

capstan and turret lathes, 128. 6.7A 

centre lathes, 101, 5.2 
Comparators, 35-51, 2.1 1-2.26 
Compound indexing, 185-7, 8.19 
Cratering, 89-90, 4.14 
Cutting, 6-8, 1.8-1.9 

speeds, 87-8, 4.10 

tools, 79-98, 4.1-4.18 

Depth of thread, 1 1 4 
Destructive testing, 2 1 
Deviation, fundamental, 59-62, 3.4 
Dial indicators, 35-8, 2.11-2.12 
Dieheads, 136-8,6.15-6.17 
Differential indexing, 187-90, 8.20 
Direct indexing, 181 



Dividing head, 180, 8.15 
Down-cut milling, 171-4, 8.5, 8.7 
Drill jigs, 150-2, 7.3-7.4 
Drilling machines, 147-66, 7.1-7.15 
Duplex boring mills, 1 19, 5.20B 
Duplex milling, 170-1, 8.3 
Dynamometers, lathe, 91, 4.16 

Electrical comparators, 42-6, 2.17-2.20 
Electrolimit gauges, 43-4, 2.17 
End measuring bars, 26-8, 2.2-2.3 
End standards, 26-33, 2.2-2.9 
Extension arms, 132-3, 6.10 

Face milling, 167-8, 8.1B 

Faceplate, 102-8, 5.6, 5.8 

Feeds and speeds, 143-4, 6.21 

Fits, 63-9, 3-7-3-9 

Fixed-bed milling, 169-70, 8.2 

Fixtures, 128-9,6.70 

Flash-butt welding, 18-20, 1.21 

Floor-to-floor time, 1 43-4 

Forces on cutting tools, 91-5, 4.15-4.16 

Form milling, 176-7, 8.1 1 

Form-relieved milling cutters, 177, 8.11-8.12 

Fundamental deviation, 59-60, 3.4 

Fusion process, 5-6, 1.6-1.7 

Gang drilling, 153-5. 7-5~7- 6 
Gap gauges, 72-3, 3.15, 3.18 
Gauge tolerance, 74-7, 3.i7~3- I 8 
Gear trains, 1 13-14, I93~4> 8.23 

Helix angle 

of miller, 191-3,8.22 

of thread, 1 15-17, 5-»7-5- 18 
High-carbon steel, 80-1, 4.3 
High-speed steel, 81-2, 4.4-4-5 
Hole-circle plates, 182-4, 8.17 
Hole production, 147-66, 7.1-7.15 
Horizontal boring, 161-3, 7.I3-7-M 
Horizontal milling, 167-8, 8.1 

Indexing, 181-90,8.16-8.20 

Inspection, 54-78. 3.1-3.18 
Interference fits, 66-7, 3.9 

Jig boring, 157- 61 . 7-9-7-" 
Jigs, drill, 150-2,7.3-7-4 



199 



200 



INDEX 



Knee turning toolholder, 13 1-2, 6.8 

Lapping of centres, 109-10, 5.11 
Lead 

of cam, 194 

of machine, 190-1,8.21 

of thread, 1 13, 5.15 
Leftward welding, 5, 1.7 
Lever comparators, 38-40, 2.13-2.14 
Limit gauges, 69-77, 3.11-3.18 
Limit systems, 55-69 
Line standards, 26 

Machinability, 93, 4.16C 
Machining times, 143-3, 6.21 
Macrostructure examination, 21 
Magnetic crack detection, 21-2, 1.22 
Mandrels, 1 10-1 1, 5.12 
Measurement, 25-53, 2.1-2.26 
Mechanical comparators, 37-42, 2.12-2. 16 
Microscope, toolmaker's, 50-1, 2.25-2.26 
Milling, 167-198, 8.1-8.25 
Multi-gauging machines, 45-6, 2.20 
Multiple-head drillers, 155 
Multiple-spindle drillers, 152-6 

Negative-rake cutting, 96-7, 4.18 
Negative-rake milling, 174-6,8.8-8.10 

Non-destructive testing, 21-2 

Operating time, 143 
Optical comparators, 46-51, 2.21-2.26 
Oxy-acetylene cutting, 6-8, 1.8-1.9 
Oxy-acetylene welding, 3-6, 1.5-1.7 

Piercing of holes, 148-50, 7.2 

Pitch of thread, 1 13, 5.15 

Plain milling, 167, 8.1A 

Plug gauges, 70-2, 3.11-3.13, 3.17 

Preferred sizes, 68-9 

Pressure on cutting tools, 93-5, 4.16 

Pressure resistance welding, 13-16, 1.15-1.17 

Profile cutting, 6-8, 1.8-1.9 

Projection welding, 16-18, 1.18-1.19 

Radial cutting, 95, 4.17 

Rake angles, 93 

Resistance welding, 13-16, 1.15-1.17 

Resultant force on tool, 92—3 

Rightward welding, 5-6, 1.7 

Ring gauges, 73, 3.16 

Roller-steady ending toolholder, 135-6, 6.14 

Roller-steady turning toolholder, 133-5, 

6. 12-6. 13 
Rollers, precision, 28, 2.5-2.7 
Rotary-table production milling, 171, 8.4 

Screw-cutting, 1 1 1-17, 5.13-5.18 
Seam welding, 15, 1.16 



Self-opening diehead, 136-8, 6.15-6.17 
Shielded-arc welding, 10-11, 1.12 
Sigma comparators, 41-2, 2.16 
Simple indexing, 181-2, 8.16 
Slab milling, 167, 8.1A 
Slip gauges, 26, 2.5-2.9 
Slotting attachments, 180, 8.14 
Spindle speeds, 129-31 
Spiral angle 

of miller, 19 1-3, 8.22 

of thread, 115-17,5.17-5.18 
Spiral milling, 190-4, 8.21-8.23 
Splined holes, 156, 7.8 
Spot welding, 13-16, 1.2, 1.15-1.17 
Starting drill, 133,6.11 
Starts, 1 12-15, 5.14-5.16 
Stellite, 82-3, 4.6, 4.10 
Stitch welding, 15-16, 1.16-1.17 
Submerged-arc welding, 11-12, 1.13 

Tangential cutting, 95-6, 4.17 
Taper-lock plug gauges, 71, 3.12 
Tapping, 138-9, 6.18 
Thread cutting 

external, 1 1 1-17, 5.13-5.18 

internal, 138-9,6.18 
Tolerance 

gauge, 74-7, 3.17-3.18 

grade, 56-8, 3.2 

standard, 58 
Tool failure, 88-9 
Tool holding 

capstan and turret lathes, 13 1-9, 6.8-6.18 

centre lathes, 100, 5.1 
Toolmaker's buttons, 102-6, 5.3-5.7 
Toolmaker's microscope, 50-1, 2.25-2.26 
Transition fits, 64-6, 3.8 
Trilock plug gauges, 71-2, 3.13-3.14 
Tungsten carbides, 83-6, 4.7-4.10 
Tungsten electrode welding, 12, 1.14A 
Turret lathes, 123-46, 6.1-6.22 
Turret tooling, 1 19-20, 5.20G 
Twisted-strip comparators, 40-1, 2.15 

Universal milling machines, 1 77-80 
Up-cut milling, 17 1-3, B.^-^B.G 

Vector diagrams, 107-8, 5.9 
Vertical boring, 117-20, 5.19-5.20 
Vertical milling attachments, 179—80, 8.13 
Visual gauging heads, 44-5, 2.19 

Welding, 1-24, 1.1-1.22 

Widia, 84 

Wigglers, 38-40, 2. 13-2. 14 

Work holding 

capstan and turret lathes, 128-9, 6.7 
centre lathes, 1 10-1 1, 5.2, 5.10-5.12 

X-ray examination, 22 



X 

o 



This volume Is one of a series of four books j 
specially written and Illustrated to cover the - 
syllabuses of Workshop Processes (T1 and 
T2) and Workshop Technology (T3 and T4) 
of the CGLI Mechanical Engineering 
Technician's course. 

The principles and applications of welding, 

measurement, Inspection, cutting toots, 

and the use of standard machine tools, 

Including centre, turret, and capstan lathes, 

and drilling and milling machines, are Q 

discussed with a simplicity of approach 

aided by the wide use of clear line diagrams. 

Only rational dimensions and quantities In 
SI units are used throughout this new 
edition. 



ft 

A 



o 
o 



7% 



621 
.75 



£1.95 net in UK isbn o 340 15762 3